[position] Implement generating pawn moves by looking up bitboards in the Library

This enables a bunch of clean up! Remove the MoveGenerationParameters and MoveList
types from move_generator::pawn.

Implement BitBoard::pawn_pushes to look up pawn pushes by square and color.

bitboard/src/bitboard.rs

@@ -44,6 +44,10 @@ impl BitBoard {
         library().pawn_attacks(sq, color)
     }
 
+    pub fn pawn_pushes(sq: Square, color: Color) -> BitBoard {
+        library().pawn_pushes(sq, color)
+    }
+
     moves_getter!(knight_moves);
     moves_getter!(bishop_moves);
     moves_getter!(rook_moves);