Add access to the slot in the lazy constructor

[bpmckinnon]

There is a specific use case where I want to cache a pointer to the newly created lazy object. If the capacity of the map does not change between the first and second call of my function, then I use the pointer instead of repeating the lookup.

Fixed an initialization order issue in ReadWriteLock

Fix the return value of lazy_emplace and lazy_emplace_with_hash.