RwLock guard lock ref recovery methods, owned guards

[ijackson]

Split off from #3928 which is (now) the same thing just for Mutex.

Open questions:

• Are these the best names? parking_lot uses fn mutex() for its mutex type but does not provide these facilities for its rwlocks. smol uses source() which is more general, leading to less variation between Mutex and RwLock.

• Currently these methods are only provided here for the owned forms of the guards, which is suboptimal as noted in Mutex guard lock ref recovery methods #3928 (comment) . However, providing them for the non-owned forms is not entirely straighrforward: Mutex guard lock ref recovery methods #3928 (comment)

[Darksonn]

Adding a new generic parameter would indeed not be possible. I am inclined to having the method on the mutex type only, similar to what parking_lot does.

[ijackson]

Adding a new generic parameter would indeed not be possible. I am inclined to having the method on the mutex type only, similar to what parking_lot does.

I think having the methods where we can conveniently provide them is better, but of course it's up to you. Please feel free to close this MR if you've decided against having this on only a subset of the rwlock guards. I guess we could always resurrect it if someone else wants them.

[Darksonn]

Closing this for now.