Remove all uses of feature(import_shadowing).

r? @erickt
This is only used in tests right now, but feel free to bring in line with `core::prelude` and/or `std::prelude` and use it everywhere in collections. But mostly in modules that don't bring anything to the prelude themselves (not `slice`, `str`, `string` and `vec` - those can't import `::prelude::*`).