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.
Implement thiserror::Error for a bunch of error types, and remove string errors
from the implementation of the command handler in explorer.
Clean up parsing of basic types all over the place.
Update Cargo files to include thiserror and anyhow.
Well… all the tests except the Peter Ellis Jones tests.
Pass around whole EnPassant types instead of pulling out just the e.p. square.
Make sure that Castling moves have their target and origin squares populated.
Add a color field to the Castle move style to make this possible.
Replace Position's en_passant_target_square() and en_passant_capture_square()
with a single en_passant() method that returns a new EnPassant struct that has
the target and capture squares for the en passant move.
