Use for_each to extend collections

This updates the `Extend` implementations to use `for_each` for many
collections: `BinaryHeap`, `BTreeMap`, `BTreeSet`, `LinkedList`, `Path`,
`TokenStream`, `VecDeque`, and `Wtf8Buf`.

Folding with `for_each` enables better performance than a `for`-loop for
some iterators, especially if they can just forward to internal
iterators, like `Chain` and `FlatMap` do.

src/libstd/sys_common/wtf8.rs

@@ -388,9 +388,7 @@ impl Extend<CodePoint> for Wtf8Buf {
         let (low, _high) = iterator.size_hint();
         // Lower bound of one byte per code point (ASCII only)
         self.bytes.reserve(low);
-        for code_point in iterator {
-            self.push(code_point);
-        }
+        iterator.for_each(move |code_point| self.push(code_point));
     }
 }