add comment regarding CoordPair performance

src/repr/coords.rs

@@ -7,6 +7,7 @@ use static_assertions::const_assert;
 // performance by about 1-2% overall
 pub type CoordAxis = u8;
 
+// using u8 for this results in a ~3-6% perf increase
 pub type CoordPairInner = u8;
 
 const_assert!(CoordPairInner::MAX as usize >= Board::BOARD_AREA as usize);