librustc: De-`@mut` `BindingRscope::anon_bindings`
librustc: De-`@mut` the `users` list in liveness
librustc: De-`@mut` `outputs` in the session
librustc: De-`@mut` `populated_external_traits` in the type context
librustc: De-`@mut` the crate cache in the crate reader