[board] Add a piece! macro

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)

board/src/lib.rs

@@ -2,6 +2,7 @@
 
 mod bitboard;
 mod moves;
+#[macro_use]
 pub mod piece;
 pub mod position;
 mod square;