… r=alexcrichton

Cleanup old issue references. One of these workarounds no longer need to be used anymore and the others are out of date.

Fix `mut_mutex_lock` when reference not ultimately mutable

When there is are multiple references where one of the references isn't mutable then this results in a false-positive for `mut_mutex_lock` as it only checks the mutability of the first reference level.

Fix this by using `peel_mid_ty_refs_is_mutable` which correctly determines whether the reference is ultimately mutable and thus whether `Mutex::get_lock()` can actually be used.

Fixes rust-lang#9854

changelog: [`mut_mutex_lock`]: No longer lints if the mutex is behind multiple references and one of those references isn't mutable