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/libsync/comm/sync.rs

@@ -426,7 +426,7 @@ impl<T> Buffer<T> {
     fn enqueue(&mut self, t: T) {
         let pos = (self.start + self.size) % self.buf.len();
         self.size += 1;
-        let prev = mem::replace(self.buf.get_mut(pos), Some(t));
+        let prev = mem::replace(&mut self.buf[pos], Some(t));
         assert!(prev.is_none());
     }
 
@@ -434,7 +434,7 @@ impl<T> Buffer<T> {
         let start = self.start;
         self.size -= 1;
         self.start = (self.start + 1) % self.buf.len();
-        self.buf.get_mut(start).take().unwrap()
+        self.buf[start].take().unwrap()
     }
 
     fn size(&self) -> uint { self.size }