New convention: active_color_ methods operate on the active color of the Board.
Methods without that prefix take a color parameter and operate on that.
Refactor opposing_sight to do this.
Implement a new method on Position that evaluates whether the active color can castle
on a given wing of the board. Then, implement making a castling move in the position.
Make a new Wing enum in the core crate to specify kingside or queenside. Replace the
Castle enum from the board crate with this one. This caused a lot of churn...
Along the way fix a bunch of tests.
Note: there's still no way to actually make a castling move in explorer.
Encapsulate castling rights within a small module. Castle provides the API to the
rest of the board package. Rights encodes the castling rights for each player.
Parameters defines castling parameters for each player.
