add comment about loop unrolling

src/repr/board.rs

@@ -291,6 +291,8 @@ impl Board {
                 let mut temp_flips = BitBoard::new();
 
                 // Expand in direction until edge or non-opponent piece
+                // PERF! tested unrolling this loop using `const_for`
+                // resulted in a 9% slowdown
                 loop {
                     current = $dir(&current, 1);
                     if current.is_empty() || !current.intersects(*opponent_board) {