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.
Now that board has a Mailbox, the implementation of this routine can be greatly
simplified. Instead of needing to iterate through all BitBoards to find the
occupied square, just consult the mailbox.
