Add basic move generation algorithms

[amogh-dambal]

The next fundamental step of building out our engine is to build a efficient, correct move generation algorithm. Generation of pseudo-legal or legal moves are both options, but generating legal moves ends up being more efficient since pseudo-legal move generators have to check for legality after generation anyway.

At a high level, we need to watch out for:

1. King attacks
2. Check evasions
3. Pinned pieces

References See https://peterellisjones.com/posts/generating-legal-chess-moves-efficiently/ for notes on how to do this.

[amogh-dambal]

Some test FEN strings as well as a perft output for testing purposes: https://gist.github.com/peterellisjones/8c46c28141c162d1d8a0f0badbc9cff9