Revision: change iter_move to into_iter

After accepting the ownership conventions RFC in a weekly meeting, the core team separately met and decided to reconsider the convention in the special case of iterators.

We felt that `for x in v.into_iter()` reads more clearly than `for x in v.iter_move()`, and since the feedback from the community/weekly meeting was essentially a tossup between the two, we decided to special-case moving iterators to `into_iter`.

This PR adjusts the RFC accordingly.