My idea was to implement traits on Piece that return sight lines, etc. This has
turned out to be much more complex than I thought it would be. Ultimately, I
don't think it's worth the effort.
This macro implements a tiny DSL for creating Pieces and PlacedPieces.
"White King" → Piece::new(Color::White, Shape::King)
"White King on E4" → PlacedPiece::new(Piece::new(Color::White, Shape::King), Square::E4)
Move position.rs to the position module and create a mod.rs.
Do the same for bitboard.rs in the bitboard modules.
Export Color, Piece, Position, and Square and use crate::Thing directly instead of referring to the symbol in the nested modules.