add comment about Move.children

src/logic/move.rs

@@ -15,6 +15,10 @@ pub struct Move {
     pub parent: Option<usize>,
 
     /// Indices of this Move's Children
+    // PERF! this accounts for ~40% of memory usage
+    // I'm thinking maybe switching to this actually being `Vec<Box<Move>>`
+    // and ditching the entire `Arena` idea, that would have
+    // cascading effects though, something to think about
     pub children: Vec<usize>,
 
     /// Has this [`Move`] already attempted to create children?