The trait will keep the `Iterator` naming, but a more concise module
name makes using the free functions less verbose. The module will define
iterables in addition to iterators, as it deals with iteration in
general.
See discussion in #8489, but this selects option 3 by adding a `Default` trait to be implemented by various basic types.
Once this makes it into a snapshot I think it's about time to start overhauling all current use-cases of `fmt!` to move towards `ifmt!`. The goal is to replace `%X` with `{}` in 90% of situations, and this commit should enable that.
