Add PeekMut::pop

A fairly common workflow is to put a bunch of stuff into a binary heap
and then mutate the top value until its empty. This both makes that a
bit more convenient (no need to save a boolean off and pop after to
avoid borrowck issues), and a bit more efficient since you only shift
once.

src/libcollectionstest/binary_heap.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 use std::collections::BinaryHeap;
-use std::collections::binary_heap::Drain;
+use std::collections::binary_heap::{Drain, PeekMut};
 
 #[test]
 fn test_iterator() {
@@ -94,6 +94,19 @@ fn test_peek_mut() {
     assert_eq!(heap.peek(), Some(&9));
 }
 
+#[test]
+fn test_peek_mut_pop() {
+    let data = vec![2, 4, 6, 2, 1, 8, 10, 3, 5, 7, 0, 9, 1];
+    let mut heap = BinaryHeap::from(data);
+    assert_eq!(heap.peek(), Some(&10));
+    {
+        let mut top = heap.peek_mut().unwrap();
+        *top -= 2;
+        assert_eq!(PeekMut::pop(top), 8);
+    }
+    assert_eq!(heap.peek(), Some(&9));
+}
+
 #[test]
 fn test_push() {
     let mut heap = BinaryHeap::from(vec![2, 4, 9]);