Add an iterate function to core::iter

Implementation by Kevin Ballard.

The function returns an Unfold iterator producing an infinite stream
of results of repeated applications of the function, starting from
the provided seed value.

src/libcoretest/iter.rs

@@ -833,3 +833,12 @@ fn test_min_max_result() {
     let r = MinMax(1i,2);
     assert_eq!(r.into_option(), Some((1,2)));
 }
+
+#[test]
+fn test_iterate() {
+    let mut it = iterate(|x| x * 2, 1u);
+    assert_eq!(it.next(), Some(1u));
+    assert_eq!(it.next(), Some(2u));
+    assert_eq!(it.next(), Some(4u));
+    assert_eq!(it.next(), Some(8u));
+}