collections: Enable IndexMut for some collections

This commit enables implementations of IndexMut for a number of collections,
including Vec, RingBuf, SmallIntMap, TrieMap, TreeMap, and HashMap. At the same
time this deprecates the `get_mut` methods on vectors in favor of using the
indexing notation.

cc #18424

src/libserialize/json.rs

@@ -1231,7 +1231,7 @@ impl Stack {
             InternalIndex(i) => { i + 1 }
             _ => { panic!(); }
         };
-        *self.stack.get_mut(len - 1) = InternalIndex(idx);
+        self.stack[len - 1] = InternalIndex(idx);
     }
 }