Replace Numba with Python + Cython

[mdekstrand]

Numba is holding back our release and continuing to be a hassle, and is also the reason why we need to maintain CSR.

Dropping the Numba dep will speed up development quite a bit, even if it slows down a few of the algorithms.

• De-Numba User KNN
• De-Numba Item KNN
• De-Numba ALS
• De-Numba FunkSVD
• Replace CSR with SciPy sparse arrays
• Make sure TF doesn't use the negative sampling utilities

[mdekstrand]

Abandoning this in favor of #374.