Release version 0.7

Changes
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- The build system has been migrated from Makefiles to Meson.

- Top-level source files and manpages have been moved into a new
  "src/" and "docs/" subdirectory, respectively.

- Support for encryption has been completely removed. Its
  security cannot be guaranteed and better solutions exist by
  e.g. writing the backup store into a LUKS volume.

- The hex conversion and blake2b hash functions have been
  included into our sources. This allowed removing the dependency
  on libsodium altogether.