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/path.rs

@@ -1551,9 +1551,7 @@ impl<P: AsRef<Path>> iter::FromIterator<P> for PathBuf {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<P: AsRef<Path>> iter::Extend<P> for PathBuf {
     fn extend<I: IntoIterator<Item = P>>(&mut self, iter: I) {
-        for p in iter {
-            self.push(p.as_ref())
-        }
+        iter.into_iter().for_each(move |p| self.push(p.as_ref()));
     }
 }