Migrate to Rust 2021 edition

refactor code to ease maintenance

make code more robust against future change

The new Pin API offers the formal guarantee that referential structs
will never move in memory. Although this does not happen in Stringsext
anyway, we use Pin to prevent any futures code changes that would break
this guarantee.

documentation: switch docbook toolchain

No changes in source code.

migrate argument parser: DocOpt-> StructOpt

New feature: "same-Unicode-block"

Requires all characters in a finding to originate from the same Unicode
block. This option helps to reduce false positives, especially when
scanning for UTF-16. When set, "`stringsext`" prints only Unicode block
homogenous strings. For example: "`-u All -n 10 -r`" finds a sequence of at
least 10 Cyrillic characters in a row or finds at least 10 Greek characters
in a row, whereas it ignores strings with randomly Cyrillic-Greek mixed
characters.  Technically this option guarantees, that all multibyte
characters of a finding - encoded as UTF-8 - start with the same leading
byte.

Release Version v2.1.1

This release contains a major rewrite of the "Stringsext"
string-extraction-tool with the following improvements and new features:

* Much faster (>30%).
* Improved Unicode-Block-Filter:
  - Search in scripts with predefined filters e.g.
    *Latin*, *Arabic*, *Syriac*, *Cyrillic*, etc and any combination of
    these.
  - Configurable custom filter.
* Improved ASCII-Filter:
  - Predefined filters e.g. *all ASCII without controls* or *all
    ASCII with white-space, without controls".
  - Custom filters with configurable sets of ASCII-codes that
    pass the filter.
* New internal "grep"-like filter, mainly useful to search paths strings.
* More detailed position indication for long strings.
* Better interface with other stream oriented tools e.g. "head",
  "tail", "sed" and "grep".
* Better handling of zero terminated (C-style) strings in large fields.
* New backend "encoding_rs".

Changes in v2.1.1

* set default `--output-line-len` to 64 characters.
  This implies: set default `input_window` width to
  2*64 bytes = 128 bytes

Release Version v2.1.0

This release contains a major rewrite of the "Stringsext"
string-extraction-tool with the following improvements and new features:

* Much faster (>30%).
* Improved Unicode-Block-Filter:
  - Search in scripts with predefined filters e.g.
    *Latin*, *Arabic*, *Syriac*, *Cyrillic*, etc and any combination of
    these.
  - Configurable custom filter.
* Improved ASCII-Filter:
  - Predefined filters e.g. *all ASCII without controls* or *all
    ASCII with white-space, without controls".
  - Custom filters with configurable sets of ASCII-codes that
    pass the filter.
* New internal "grep"-like filter, mainly useful to search paths strings.
* More detailed position indication for long strings.
* Better interface with other stream oriented tools e.g. "head",
  "tail", "sed" and "grep".
* Better handling of zero terminated (C-style) strings in large fields.
* New backend "encoding_rs".

Changes in v2.1.0

* ASCII-filter `None` also suppresses the Null character (0x00).

Release Version v2.0.0

This release contains a major rewrite of the "Stringsext"
string-extraction-tool with the following improvements and new features:

* Much faster (>30%).
* Improved Unicode-Block-Filter:
  - Search in scripts with predefined filters e.g.
    *Latin*, *Arabic*, *Syriac*, *Cyrillic*, etc and any combination of
    these.
  - Configurable custom filter.
* Improved ASCII-Filter:
  - Predefined filters e.g. *all ASCII without controls* or *all
    ASCII with white-space, without controls".
  - Custom filters with configurable sets of ASCII-codes that
    pass the filter.
* New internal "grep"-like filter, mainly useful to search paths strings.
* More detailed position indication for long strings.
* Better interface with other stream oriented tools e.g. "head",
  "tail", "sed" and "grep".
* Better handling of zero terminated (C-style) strings in large fields.
* New backend "encoding_rs".

Release Version v1.99.5 (2.0.0pre5)

This release contains a major rewrite of the "Stringsext"
string-extraction-tool with the following improvements and new features:

* Much faster (>30%).
* Improved Unicode-Block-Filter:
  - Search in scripts with predefined filters e.g.
    *Latin*, *Arabic*, *Syriac*, *Cyrillic*, etc and any combination of
    these.
  - Configurable custom filter.
* Improved ASCII-Filter:
  - Predefined filters e.g. *all ASCII without controls* or *all
    ASCII with white-space, without controls".
  - Custom filters with configurable sets of ASCII-codes that
    pass the filter.
* New internal "grep"-like filter, mainly useful to search paths strings.
* More detailed position indication for long strings.
* Better interface with other stream oriented tools e.g. "head",
  "tail", "sed" and "grep".
* Better handling of zero terminated (C-style) strings in large fields.
* New backend "encoding_rs".

Release Version v1.99.4 (2.0.0pre4)

This release contains a major rewrite of the "Stringsext"
string-extraction-tool with the following improvements and new features:

* Much faster (>30%).
* Improved Unicode-Block-Filter:
  - Search in scripts with predefined filters e.g.
    *Latin*, *Arabic*, *Syriac*, *Cyrillic*, etc and any combination of
    these.
  - Configurable custom filter.
* Improved ASCII-Filter:
  - Predefined filters e.g. *all ASCII without controls* or *all
    ASCII with white-space, without controls".
  - Custom filters with configurable sets of ASCII-codes that
    pass the filter.
* New internal "grep"-like filter, mainly useful to search paths strings.
* More detailed position indication for long strings.
* Better interface with other stream oriented tools e.g. "head",
  "tail", "sed" and "grep".
* Better handling of zero terminated (C-style) strings in large fields.
* New backend "encoding_rs".

Release Version v1.99.2 (2.0.0pre2)

This release contains a major rewrite of the "Stringsext"
string-extraction-tool with the following improvements and new features:

* Much faster (>30%).
* Improved Unicode-Block-Filter:
  - Search in scripts with predefined filters e.g.
    *Latin*, *Arabic*, *Syriac*, *Cyrillic*, etc and any combination of
    these.
  - Configurable custom filter.
* Improved ASCII-Filter:
  - Predefined filters e.g. *all ASCII without controls* or *all
    ASCII with white-space, without controls".
  - Custom filters with configurable sets of ASCII-codes that
    pass the filter.
* New internal "grep"-like filter, mainly useful to search paths strings.
* More detailed position indication for long strings.
* Better interface with other stream oriented tools e.g. "head",
  "tail", "sed" and "grep".
* Better handling of zero terminated (C-style) strings in large fields.
* New backend "encoding_rs".

Release Version v1.99.1 (2.0.0pre1)

This contains a major rewrite of the "Stringsext" string-extraction-tool
with the following improvements and new features:

* Much faster (>30%).
* Improved Unicode-Block-Filter:
  - Search in scripts with predefined filters e.g.
    *Latin*, *Arabic*, *Syriac*, *Cyrillic*, etc and any combination of
    these.
  - Configurable custom filter.
* Improved ASCII-Filter:
  - Predefined filters e.g. *all ASCII without controls* or *all
    ASCII with white-space, without controls".
  - Custom filters with configurable sets of ASCII-codes that
    pass the filter.
* New internal "grep"-like filter, mainly useful to search paths strings.
* More detailed position indication for long strings.
* Better interface with other stream oriented tools e.g. "head",
  "tail", "sed" and "grep".
* Better handling of zero terminated (C-style) strings in large fields.
* New backend "encoding_rs".

code freeze for version 1

important bugfix, upgrade used library

Under certain circumstances the input data was not read until the end.
In the worst case the last 6150 bytes of a file or stream were not
analysed.

Follow API change in memmap 0.6.

replace depreciated library

This migrates from `rustc_serialize` to `serde`.

dual license stringsext: Apache 2.0 or MIT

no changes in source code

minor code cleanups and documentation updates

fix typos in doc and add custom error type

new option: --print-file-name, do not abort when not readable

- New option: --print-file-name: Print the name of the file before each string.

- When one of the input files is not readable, skip with a warning
  and continue.