Allow mixing `Gc` pointers from different `CollectorId`s

[Techcable]

This should work cleanly for CollectorIds of different types. Mixing multiple collector ids of the same type is unlikely to be supported right now.

I already have a doctest demonstrating the expected behavior.

All that remains is to get the procedural drive to support it, then test mixing the 'simple' collector with the 'epsilon' collector. The main difficulty is breaking the derive macro's current assumption that there is a single 'gc lifetime. With multiple collector ids, there are likely to be multiple GC lifetimes.