Support for accessing host environment variables

[cuviper]

Miri currently starts with an empty environment, as noted on Evaluator::env_vars:

/// Miri does not expose env vars from the host to the emulated program.

It would be nice to have some control over this from the command line, as suggested on reddit
by /u/0b_0101_001_1010.

Possible inspirations:

• docker run -e / --env / --env-file (ref)
• sudo -E / --preserve-env (ref)

[RalfJung]

Related to #800.

[RalfJung]

With #894, you can pass -Zmiri-enable-communication to let the interpreted program access host environment variables.

The name of that flag is still up for debate; let's discuss that in #800 though.