Suggest using iter() or into_iter() for Vec

We cannot do that for `&Vec` because `#[rustc_on_unimplemented]` is limited (it does not clean generic instantiation for references, only for ADTs).

Author: ChayimFriedman2

core/src/iter/traits/iterator.rs

@@ -40,6 +40,10 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
         label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
     ),
     on(_Self = "&[]", label = "`{Self}` is not an iterator; try calling `.iter()`"),
+    on(
+        _Self = "std::vec::Vec<T, A>",
+        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
+    ),
     on(
         _Self = "&str",
         label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"