Cryptg

Moved away from `cffi` to use `grammers-crypto` as the backing implementation and `pyo3` in order to provide Python bindings.

Stopped using [C / C++ Python Extensions](https://docs.python.org/3/extending/index.html) for CPython in favour of [`cffi`](https://cffi.readthedocs.io), which is the correct way of building portable C extensions for different interpreters.

Now requires `libffi-dev` for building. For CPython, `python3-dev` is also necessary.