Add Entry::key

This method was present on both variants of Entry, but not the enum

cc #32281

Author: sfackler

src/libcollections/btree/map.rs

@@ -1522,6 +1522,15 @@ impl<'a, K: Ord, V> Entry<'a, K, V> {
             Vacant(entry) => entry.insert(default()),
         }
     }
+
+    /// Returns a reference to this entry's key.
+    #[unstable(feature = "map_entry_keys", issue = "32281")]
+    pub fn key(&self) -> &K {
+        match *self {
+            Occupied(ref entry) => entry.key(),
+            Vacant(ref entry) => entry.key(),
+        }
+    }
 }
 
 impl<'a, K: Ord, V> VacantEntry<'a, K, V> {

src/libstd/collections/hash/map.rs

@@ -1533,6 +1533,15 @@ impl<'a, K, V> Entry<'a, K, V> {
             Vacant(entry) => entry.insert(default()),
         }
     }
+
+    /// Returns a reference to this entry's key.
+    #[unstable(feature = "map_entry_keys", issue = "32281")]
+    pub fn key(&self) -> &K {
+        match *self {
+            Occupied(ref entry) => entry.key(),
+            Vacant(ref entry) => entry.key(),
+        }
+    }
 }
 
 impl<'a, K, V> OccupiedEntry<'a, K, V> {