This makes Position immutable, even if declared mut. I think this will also make
it easier to build positions going forward.
Moving to a PieceBitBoards struct also required a lot of places that return owned
BitBoards to return refs. This is basically fine except for adding a few more &
here and there.
This was a huge refactoring project. All the move generators and a bunch of
BitBoard stuff needed to be updated.
This struct computes and stores piece moves per square to speed up computation
of move lists.
Initialize a `static mut` instance once via std::sync::Once, and return a static
reference to it. So far, it computes king and knight moves.
Make BitBoard::empty() and MoveLibrary::new() const functions, enabling static
construction of the MOVE_LIBRARY instance without needing to wrap it in an
Option.
