First attempt at performance optimization

[john-science]

Right now the .pxd files work, and the Cythonized version is certainly faster than the pure Python code. But I haven't done any optimization for performance.

I bet there are tweaks that could be made to the pxd files, and maybe the Python code, that would make these algorithms significantly faster.

I believe a good place to start would be working with a single problem chain, say Prims for the generator and Backtracking for the solver.

1. Make it work
2. Make it right
3. Make it fast

I believe (1) and (2) above are doing well. Maybe it's time to try (3).