iter: remove `to_owned_vec`

This needs to be removed as part of removing `~[T]`. Partial type hints
are now allowed, and will remove the need to add a version of this
method for `Vec<T>`. For now, this involves a few workarounds for
partial type hints not completely working.
auto merge of #13090 : thestinger/rust/iter, r=Aatch

This has been rendered obsolete by partial type hints. Since the `~[T]`
type is in the process of being removed, it needs to go away.