Remove Cython?

[jcrist]

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

[rzu512]

If the cython part and c part are separated, people can use the c part in other projects.