C bindings crate

[masklinn]

I initially started working on one for CFFI as I believed pyo3 wasn't compatible with pypy or graal, but turns out it is so that's not necessary.

Might still be useful for other languages, but I can't say I have much confidence in writing a good C API myself, and have little need to besides¹.

Footnotes

1. although a cffi version would likely be more efficient than cpyext for pypy and graal, the perf gain of just cpyext compared to pure python is so huge it's not really relevant ↩