Remove Cython?

Currently Cython is used just as a thin wrapper around C code. This is a bit tricky, since our C code also touches the python/numpy c-api, and so reference counting must remain consistent through the two layers. In my experience developing this I've started to wonder if removing cython in favor of a straight C-extension might make more sense. This would also generate a much smaller binary, as Cython generated modules have a fair bit of bloat.

Modules to conver

- [ ] Statistics (I'll start with this one, as it'll be the easiest)
- [ ] TDigest
- [ ] Space Saving