Statically assert zero-cost invariants

Since some or all of those invariants will come in handy to ensure we
use as little memory as possible, to maximize the speed of the move
generation later on.

src/board/bitboard/mod.rs

@@ -1,4 +1,6 @@
 use super::Square;
+use crate::utils::static_assert;
+
 mod iterator;
 use iterator::*;
 
@@ -63,6 +65,9 @@ impl Bitboard {
     }
 }
 
+// Ensure zero-cost (at least size-wise) wrapping.
+static_assert!(std::mem::size_of::<Bitboard>() == std::mem::size_of::<u64>());
+
 impl Default for Bitboard {
     fn default() -> Self {
         Self::EMPTY