The module std::env can be used for obtaining and setting environment
variables, the home directory, the temporary directory, and more. For
example, one can obtain environment variable values as follows:

    import std::env

    env['HOME'] # => '/home/yorickpeterse'

You can also set the value of a variable:

    import std::env

    env['HOME'] = '/home/foo'

Removing variables is also possible:

    import std::env

    env.remove('HOME') # => Nil

Or obtain the home directory:

    import std::env

    env.home_directory # => '/home/yorickpeterse'

You can also obtain and set the working directory:

    import std::env

    try! env.working_directory          # => '/home/yorickpeterse'
    try! env.working_directory = '/tmp' # => '/tmp'

Arguments can be retrieved using `std::env.arguments`:

    import std::env

    env.arguments # => ['foo', 'bar']

== Executable changes

The "inko" executable has been modified to pass additional commandline
arguments to IVM. IVM in turn has been modified to expose these to the
VM instructions. This requires us to explicitly store the passed
arguments in a vm::state::State, as Rust's std::env::args() is immutable
_and_ includes _all_ arguments (the bytecode file to execute, IVM
options, etc).

The arguments passed via the CLI are all interned, removing the need for
allocating (potentially many) strings every time `std::env.arguments` is
executed.

Fixes #136