sync: add a rwlock() method to owned RwLock guards

[r3v2d0g]

Adds a rwlock() method returning a reference to the original Arc<RwLock> to the Owned* guards.

Motivation

Mutex guards and Semaphore permits already have similar methods.

Solution

A similar, rwlock(), method is added to the Owned* guards. The "normal" guards don't actually hold a reference to the original RwLock and I don't think that it can be added to them without changing these types' definitions.

[Darksonn]

This is reasonable enough, but if we are adding it, then I would like us to be consistent and add it to all of the guards. I am okay with replacing the &Semaphore field with an &RwLock<T> field.

[r3v2d0g]

This is reasonable enough, but if we are adding it, then I would like us to be consistent and add it to all of the guards. I am okay with replacing the &Semaphore field with an &RwLock<T> field.

Although replacing the &Semaphore with an &RwLock<T> would work for RwLockWriteGuard because it is generic over the same T as the RwLock<T>, it wouldn't for RwLockMappedWriteGuard and RwLockReadGuard because both of these are generic over a type which might or might not be the same as the RwLock (RwLockWriteGuard<'a, T>::map returns RwLockMappedWriteGuard<'a, U> and RwLockReadGuard<'a, T>::map returns RwLockReadGuard<'a, U>).

The only way to make this work (AFAIK) would be to update RwLockMappedWriteGuard's signature from RwLockMappedWriteGuard<'a, U> to RwLockMappedWriteGuard<'a, T, U> where either T or U is the type wrapped by the RwLock, and to update RwLockReadGuard's signature to be similar to OwnedRwLockReadGuard, that is RwLockReadGuard<'a, T, U> where by default U = T.

Hope this all makes sense!

[mox692]

Looks good to me.