Add more introspection info for constructors

[jakemac53]

It would be good to have isFactory, isConst, and then in the case of redirecting factory constructors it would be good if we could get the name.

The last bit is a bit interesting as we may want to allow providing an implementation of an external factory constructor, which is actually just to make it a redirecting constructor. That could get added in the last phase and then wouldn't be available in earlier phases etc, so we should thing about the consequences of that. Likely we can just say that is OK though.