Reimplement Square as an enum; implement Rank and File enums

Replace the Square struct with an enum. This implementation is based on this one:

https://github.com/analog-hors/magic-bitboards-demo/blob/main/types/src/square.rs

This reduces a lot of code needed to construct squares, ranks, and files.

board/src/lib.rs

@@ -11,4 +11,4 @@ mod tests;
 
 pub use moves::Move;
 pub use position::Position;
-pub use square::Square;
+pub use square::{File, Rank, Square};