Remove _hmmc.pyx; rewrite functions from this file as python+numpy

On my tests, this makes .fit (with 100 states) 3x faster, .eval (with 1000 states) 20% faster, .decode_viterbi (with 1000 states) 7x faster.

It might be possible to write a better Cython extension, so I didn't inline functions extracted from _hmmc.pyx to _BaseHMM methods.