Add two small BitBoard slices that represent kingside and queenside squares per
color.
Add doc comments to DARK_SQUARES and LIGHT_SQUARES.
Add getters on BitBoard for getting a boardside bitboard.
Clean up imports. Import the whole library module and refer to library things in
BitBoard by path.
This enables a bunch of clean up! Remove the MoveGenerationParameters and MoveList
types from move_generator::pawn.
Implement BitBoard::pawn_pushes to look up pawn pushes by square and color.
