we don't support range-based iteration yet because it's a huge hassle to
make c++ and rust iteration styles meet in any reasonable way. this is a
good start to make rust collections *actually* usable though, so here we
go. `into_iter` is not explicitly supported, but can be used regardless.
Change-Id: Iebb76c9409059bc81623e5be81ac2656b1ec5138
