[board] Implement a BitBoard MoveLibrary

This struct computes and stores piece moves per square to speed up computation
of move lists.

Initialize a `static mut` instance once via std::sync::Once, and return a static
reference to it. So far, it computes king and knight moves.

Make BitBoard::empty() and MoveLibrary::new() const functions, enabling static
construction of the MOVE_LIBRARY instance without needing to wrap it in an
Option.

board/src/bitboard/bitboard.rs

@@ -1,6 +1,6 @@
 // Eryn Wells <eryn@erynwells.me>
 
-use super::library::{FILES, RANKS};
+use super::library::{library, FILES, RANKS};
 use super::BitScanner;
 use crate::Square;
 use std::fmt;
@@ -10,7 +10,7 @@ use std::ops::{BitAnd, BitAndAssign, BitOr, BitOrAssign, Not};
 pub(crate) struct BitBoard(pub(super) u64);
 
 impl BitBoard {
-    pub fn empty() -> BitBoard {
+    pub const fn empty() -> BitBoard {
         BitBoard(0)
     }
 
@@ -28,6 +28,10 @@ impl BitBoard {
         FILES[file]
     }
 
+    pub fn knight_moves(sq: Square) -> BitBoard {
+        library().knight_moves(sq)
+    }
+
     pub fn from_square(sq: Square) -> BitBoard {
         BitBoard(1 << sq.index())
     }