Add version command to cryptol-remote-api

[m-yac]

This PR adds a version command to the cryptol-remote-api and its Python interface. For example:

>>> connection.version()
CryptolVersionInfo(
  rpc_version='3.0.1.99',
  version='3.0.0.99',
  commit_hash='8c811b06a435f5009e8b75b36ecc8fc220e4d6bd',
  commit_branch='version-python-cmd',
  commit_dirty=True,
  ffi_enabled=True)

See the docs for the command in Version.hs or the docstring of CryptolVersionInfo in the Python API for descriptions of these fields.

Resolves #1544