make way for a new iter module
auto merge of #6056 : thestinger/rust/iter, r=graydon

The existing adaptors like `map` in the `iter` module are very flawed because they only work for `BaseIter` implementations. There are many internal iterator implementations in the standard library like the set methods (`difference`, `symmetric_difference`, `intersection`, `union`) and the `range` functions that only share the `for` loop protocol in common.

The internal iterator adaptors should be implemented to work on any implementation of that protocol, rather than just a method called `each` taking `&self`.

This just moves `iter.rs` to `old_iter.rs` and begins work on documenting and implementing a nicer module.