[board] Create a bitboard! macro and BitBoardBuilder

The Builder enables cleanly building a BitBoard out of squares.
The macro lets you create a BitBoard from a simple list of coordinates:

bitboard!(A1, B2, C3, D4, …)

board/src/lib.rs

@@ -1,5 +1,6 @@
 // Eryn Wells <eryn@erynwells.me>
 
+#[macro_use]
 mod bitboard;
 mod display;
 mod moves;