[board] Reorganize bitboard and position modules and export some symbols from the crate

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.

board/src/position/mod.rs

@@ -0,0 +1,5 @@
+mod pieces;
+mod position;
+
+pub use pieces::Pieces;
+pub use position::Position;