Add conversions for `&CStr` and `CString`.

[davidhewitt]

I just noticed that we don't have conversions for std::ffi::CStr or std::ffi::CString.

I think these should be totally possible and work very similarly to the existing conversions for &str / String.

PyUnicode_AsUTF8AndSize guarantees that there is a trailing NUL at the end (after the size bytes of text data). However due to the fact that there may be interior NUL bytes, we would probably need to use CStr::from_bytes_with_nul and fail the conversion if any interior nuls.

[fbrv]

Still available? I’m new to the project and would like to give it a go

[davidhewitt]

Please go for it!