Use min_specialization in libcore Getting `TrustedRandomAccess` to work is the main interesting thing here. - `get_unchecked` is now an unstable, hidden method on `Iterator` - The contract for `TrustedRandomAccess` is made clearer in documentation - Fixed a bug where `Debug` would create aliasing references when using the specialized zip impl - Added tests for the side effects of `next_back` and `nth`. closes #68536