std: Implement Clone for VecIterator and iterators using it

The theory is simple, the immutable iterators simply hold state
variables (indicies or pointers) into frozen containers. We can freely
clone these iterators, just like we can clone borrowed pointers.

VecIterator needs a manual impl to handle the lifetime struct member.

src/libstd/vec.rs

@@ -2232,6 +2232,10 @@ iterator!{impl VecIterator -> &'self T}
 double_ended_iterator!{impl VecIterator -> &'self T}
 pub type VecRevIterator<'self, T> = InvertIterator<&'self T, VecIterator<'self, T>>;
 
+impl<'self, T> Clone for VecIterator<'self, T> {
+    fn clone(&self) -> VecIterator<'self, T> { *self }
+}
+
 //iterator!{struct VecMutIterator -> *mut T, &'self mut T}
 /// An iterator for mutating the elements of a vector.
 pub struct VecMutIterator<'self, T> {
@@ -2244,6 +2248,7 @@ double_ended_iterator!{impl VecMutIterator -> &'self mut T}
 pub type VecMutRevIterator<'self, T> = InvertIterator<&'self mut T, VecMutIterator<'self, T>>;
 
 /// An iterator that moves out of a vector.
+#[deriving(Clone)]
 pub struct VecConsumeIterator<T> {
     priv v: ~[T],
     priv idx: uint,
@@ -2270,6 +2275,7 @@ impl<T> Iterator<T> for VecConsumeIterator<T> {
 }
 
 /// An iterator that moves out of a vector in reverse order.
+#[deriving(Clone)]
 pub struct VecConsumeRevIterator<T> {
     priv v: ~[T]
 }
@@ -3185,6 +3191,17 @@ mod tests {
         assert_eq!(xs.mut_rev_iter().size_hint(), (5, Some(5)));
     }
 
+    #[test]
+    fn test_iter_clone() {
+        let xs = [1, 2, 5];
+        let mut it = xs.iter();
+        it.next();
+        let mut jt = it.clone();
+        assert_eq!(it.next(), jt.next());
+        assert_eq!(it.next(), jt.next());
+        assert_eq!(it.next(), jt.next());
+    }
+
     #[test]
     fn test_mut_iterator() {
         use iterator::*;