[board] Implement a pawn move generator

Holy heck I went on a *journey* here. Ultimately, I needed to implement my own
index-based iterator instead of using the Vec's Iterator.

This type establishes some patterns I want to carry forward to other move
generators.

1. The use of a Parameters struct to fully parameterize the move generation
   per-color. That lets these types only need a single color-based branch
2. A list of move lists, one list for each of captures, promotions, and quiet
   moves.
3. An index-based move iterator.
4. Separate impl for generating bitboard representations of these moves

Additional changes:

- Implement BitBoard::from_square()
- Implement a Square::e5() for tests

This class doesn't implement en passant yet. It also doesn't yet have tests for
the bitboard stuff.

board/src/bitboard/bitboard.rs

@@ -28,6 +28,10 @@ impl BitBoard {
         FILES[file]
     }
 
+    pub fn from_square(sq: Square) -> BitBoard {
+        BitBoard(1 << sq.index())
+    }
+
     pub fn is_empty(&self) -> bool {
         self.0 == 0
     }