Crypto: continued

Continuation of #1854.

TODO:

- [x] Remove crypto code from `__init__.py`.
- [x] Remove `.directory` file.
- [x] Review the code and documentation.
- [x] Fix the doctests to match the Matrix printing in master.
- [x] Add the crypto module to the Sphinx documentation.
- [x] Docstring formatting/cleanup.
- [x] Put imports at the top of crypto.py.
- [x] Check coverage: 99% (only verbose prints are not covered by tests)
- [x] PEP8 cleanup (spacing, multiple statements on one line)
- [x] `matrix_inverse_mod` probably belongs elsewhere.
- [x] discuss if `verbose` option of `encipher_bifid5` is really needed