Add white castling for both wings. Found some bugs in how king sight is computed
while writing these.
In order for the king to perform the castle, the Movement bitboard needs to return
the two squares the king can castle to. That means anytime movement is calculated
for the king, the (relatively expensive) castling evaluation needs to happen.
Write two new helper static functions to create a Move for castling and promotion
moves. Since structs cannot have any functions with the same name, the two methods
that return properties related to those moves (Move::castle and Move::promotion)
need to be renamed. They're now called Move::castle_wing and Move::promotion_shape.
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.
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.
