BinaryHeap: Simplify sift down

Sift down was doing all too much work: it can stop directly when the
current element obeys the heap property in relation to its children.

In the old code, sift down didn't compare the element to sift down at
all, so it was maximally sifted down and relied on the sift up call to
put it in the correct location.

This should speed up heapify and .pop().

Also rename Hole::removed() to Hole::element()

src/libcollections/binary_heap.rs

@@ -521,29 +521,30 @@ fn sift_up(&mut self, start: usize, pos: usize) {
 
             while hole.pos() > start {
                 let parent = (hole.pos() - 1) / 2;
-                if hole.removed() <= hole.get(parent) { break }
+                if hole.element() <= hole.get(parent) { break; }
                 hole.move_to(parent);
             }
         }
     }
 
-    fn sift_down_range(&mut self, mut pos: usize, end: usize) {
-        let start = pos;
+    /// Take an element at `pos` and move it down the heap,
+    /// while its children are larger.
+    fn sift_down_range(&mut self, pos: usize, end: usize) {
         unsafe {
             let mut hole = Hole::new(&mut self.data, pos);
             let mut child = 2 * pos + 1;
             while child < end {
                 let right = child + 1;
+                // compare with the greater of the two children
                 if right < end && !(hole.get(child) > hole.get(right)) {
                     child = right;
                 }
+                // if we are already in order, stop.
+                if hole.element() >= hole.get(child) { break; }
                 hole.move_to(child);
                 child = 2 * hole.pos() + 1;
             }
-
-            pos = hole.pos;
         }
-        self.sift_up(start, pos);
     }
 
     fn sift_down(&mut self, pos: usize) {
@@ -605,7 +606,7 @@ fn pos(&self) -> usize { self.pos }
 
     /// Return a reference to the element removed
     #[inline(always)]
-    fn removed(&self) -> &T {
+    fn element(&self) -> &T {
         self.elt.as_ref().unwrap()
     }