OwnedMutexGuard is missing a mapped version

[craftytrickster]

I would like for .map to exist on the OwnedMutexGuard the same way it currently exists on MutexGuard.

[Darksonn]

We've had soundness issues due to mapping mutex guards in the past (see e.g. #2472, #3344), so I would like to first verify that this addition is sound.

@Amanieu Do you know whether there is any issue with adding a method for mapping on our owned mutex guard? We already have it on the non-owned guard.

[Amanieu]

OwnedMutexGuard::mutex gives access to the original Arc<Mutex<T>>, which won't work after mapping. So you do need a separate type.

[Darksonn]

Ok, that makes sense. We will go for a new separate type.

[Darksonn]

I figured out why we don't already have this: it's super annoying to implement.