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/square.rs

@@ -1,4 +1,5 @@
 use super::{Bitboard, File, Rank};
+use crate::utils::static_assert;
 
 /// Represent a square on a chessboard. Defined in the same order as the
 /// [Bitboard](crate::board::Bitboard) squares.
@@ -185,6 +186,9 @@ impl std::ops::Sub<Bitboard> for Square {
     }
 }
 
+// Ensure that niche-optimization is in effect.
+static_assert!(std::mem::size_of::<Option<Square>>() == std::mem::size_of::<Square>());
+
 #[cfg(test)]
 mod test {
     use super::*;