Add MappedArcMutexGuard to mirror MappedMutexGuard

[dflemstr]

Add a new type, MappedArcMutexGuard, that mirrors the API of MappedMutexGuard but with an owned Arc<> reference to the underlying mutex while locked.

I didn't find any tests covering MappedMutexGuard, so I didn't change the status quo. Instead, I copied the exact same implementation as for MappedMutexGuard but with lifetimes removed and &'a R references to the raw mutex replaced with Arc<Mutex<T, R>>. I believe this should make the new type at least as sound as the previous MappedMutexGuard.

Unfortunately, since we only have an Arc around Mutex, but not the raw mutex type R, we cannot type erase the wrapped mutex and need to retain the original type T in MappedArcMutexGuard<R, T, U> (as opposed to MappedArcMutexGuard<R, U>).

The name was chosen based on the existing pattern of MappedFairMutexGuard to arrive at MappedArcMutexGuard (as opposed to e.g. ArcMappedMutexGuard or whatever).

[dflemstr]

I'm going to iterate a bit on the implementation of this PR and test it in various tricky production cases to make sure it is sound, so moving into Draft state

[dflemstr]

Gonna do some more testing but I think this PR is ready to get new eyes on it.