[bitboard] Make the bitboard crate more crate-like

Export symbols needed to use BitBoard and BitBoardBuilder.
Fix build errors.

bitboard/src/library.rs

@@ -1,6 +1,6 @@
 // Eryn Wells <eryn@erynwells.me>
 
-use super::BitBoard;
+use crate::BitBoard;
 use chessfriend_core::{Direction, Square};
 use std::sync::Once;