Refactor to present C and Python implementations as separate modules

Refactor to present the C and Python implementations as separate modules so that we can test them without reload()ing modules (which is too magical for my taste). Proposed layout:

• igwn_segments/_segments.so: C implementation, same as it is now
• igwn_segments/_segments_py.py: Python implementation
• igwn_segments/__init__.py: from ._segments import *