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/liballoc/collections/binary_heap.rs

@@ -1177,9 +1177,7 @@ impl<T: Ord> BinaryHeap<T> {
 
         self.reserve(lower);
 
-        for elem in iterator {
-            self.push(elem);
-        }
+        iterator.for_each(move |elem| self.push(elem));
     }
 }
 

src/liballoc/collections/btree/map.rs

@@ -1727,9 +1727,9 @@ impl<K: Ord, V> FromIterator<(K, V)> for BTreeMap<K, V> {
 impl<K: Ord, V> Extend<(K, V)> for BTreeMap<K, V> {
     #[inline]
     fn extend<T: IntoIterator<Item = (K, V)>>(&mut self, iter: T) {
-        for (k, v) in iter {
+        iter.into_iter().for_each(move |(k, v)| {
             self.insert(k, v);
-        }
+        });
     }
 }
 

src/liballoc/collections/btree/set.rs

@@ -883,9 +883,9 @@ impl<'a, T> IntoIterator for &'a BTreeSet<T> {
 impl<T: Ord> Extend<T> for BTreeSet<T> {
     #[inline]
     fn extend<Iter: IntoIterator<Item = T>>(&mut self, iter: Iter) {
-        for elem in iter {
+        iter.into_iter().for_each(move |elem| {
             self.insert(elem);
-        }
+        });
     }
 }
 

src/liballoc/collections/linked_list.rs

@@ -1107,9 +1107,7 @@ impl<T> Extend<T> for LinkedList<T> {
 
 impl<I: IntoIterator> SpecExtend<I> for LinkedList<I::Item> {
     default fn spec_extend(&mut self, iter: I) {
-        for elt in iter {
-            self.push_back(elt);
-        }
+        iter.into_iter().for_each(move |elt| self.push_back(elt));
     }
 }
 

src/liballoc/collections/vec_deque.rs

@@ -2677,9 +2677,7 @@ impl<'a, T> IntoIterator for &'a mut VecDeque<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A> Extend<A> for VecDeque<A> {
     fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T) {
-        for elt in iter {
-            self.push_back(elt);
-        }
+        iter.into_iter().for_each(move |elt| self.push_back(elt));
     }
 }