make `MutItems` iterator sound again

This become `Pod` when it was switched to using marker types.
auto merge of #12694 : thestinger/rust/mut_iter, r=huonw

This become `Pod` when it was switched to using marker types.
mark the `map` method on Vec<T> as deprecated

This exists for the sake of compatibility during the ~[T] -> Vec<T>
transition. It will be removed in the future.
auto merge of #12697 : thestinger/rust/vec, r=huonw

This exists for the sake of compatibility during the ~[T] -> Vec<T>
transition. It will be removed in the future.