Add index methods to OrderedMap and OrderedSet.

This replaces the `RandomAccessIterator` impls removed in e215273.

See servo/string-cache#80

phf/src/ordered_map.rs

@@ -81,6 +81,12 @@ impl<K, V> OrderedMap<K, V> {
         self.get_internal(key).map(|(i, _)| i)
     }
 
+    /// Returns references to both the key and values at an index
+    /// within the list used to initialize the ordered map. See `.get_index(key)`.
+    pub fn index(&self, index: usize) -> Option<(&K, &V)> {
+        self.entries.get(index).map(|&(ref k, ref v)| (k, v))
+    }
+
     /// Like `get`, but returns both the key and the value.
     pub fn get_entry<T: ?Sized>(&self, key: &T) -> Option<(&K, &V)>
             where T: Eq + PhfHash, K: Borrow<T> {

phf/src/ordered_set.rs

@@ -54,6 +54,12 @@ impl<T> OrderedSet<T> {
         self.map.get_index(key)
     }
 
+    /// Returns references to both the key and values at an index
+    /// within the list used to initialize the ordered map. See `.get_index(key)`.
+    pub fn index(&self, index: usize) -> Option<&T> {
+        self.map.index(index).map(|(k, &())| k)
+    }
+
     /// Returns true if `value` is in the `Set`.
     pub fn contains<U: ?Sized>(&self, value: &U) -> bool where U: Eq + PhfHash, T: Borrow<U> {
         self.map.contains_key(value)