[RFC][Accepted] weakrefs

Morning internals, The vote for weakrefs is closed and has been accepted. Because of feedback received very late during voting from non-voting contributors/community members, the implementation will be slightly different to that in the RFC - it was pointed out that it's more useful if there is only one weakref for every object, and it happens to simplify the internal implementation also. The only difference is the __construct will be replaced with a named constructor so that we may achieve this: It will still have the same interface as accepted, but with a different name for constructor and simpler implementation. Cheers Joe