Add a comment about implementing Keys in terms of Range in the future.

lib/entity/src/keys.rs

@@ -1,4 +1,7 @@
 //! A double-ended iterator over entity references.
+//!
+//! When `std::iter::Step` is stablized, `Keys` could be implemented as a wrapper around
+//! `std::ops::Range`, but for now, we implment it manually.
 
 use EntityRef;
 use std::marker::PhantomData;