Dereference operator for Proxy class in smart_lock

[FerdinandSpitzschnueffler]

Brief feature description

Sometimes it would be nice to access the underlying object of the Proxy object of a smart_lock, e.g. via the dereference operator.

concurrent::smart_lock<std::vector<int>> threadSafeVector;
...
auto guardedVector = threadSafeVector.getScopeGuard();
doSomethingWithVector(*guardedVector);