Fix reverse iteration in cretonne-entity's Iter.

A `DoubleEndedIterator` needs to track both a forward position and a
reverse position, so that `next_back()` can pop from the back of the
sequence.

lib/entity/src/keys.rs

@@ -11,11 +11,11 @@ pub struct Keys<K: EntityRef> {
 }
 
 impl<K: EntityRef> Keys<K> {
-    /// Create a `Keys` iterator that visits `count` entities starting from 0.
-    pub fn new(count: usize) -> Self {
+    /// Create a `Keys` iterator that visits `len` entities starting from 0.
+    pub fn with_len(len: usize) -> Self {
         Self {
             pos: 0,
-            rev_pos: count,
+            rev_pos: len,
             unused: PhantomData,
         }
     }