Revisit opaque type handling in Strong/WeakPtr

[gorlak]

It currently uses a void* to allow safe instantiation on forward declarations. This is of questionable use and also breaks seeing object data in the debugger. Consider re-adding a typed pointer and changing includes to bring in full definitions of objects when we need a pointer to that type. This will be more costly compile-wise, but debugging will be much easier.

[gorlak]

Curious what @aclysma's thoughts on on this.

[aclysma]

I think this would be a good change. The only thing that would prevent me from being behind this change is if this made it impossible to make a A have a strong reference to B and B have a strong reference to A. We might still need an opaque version of StrongPtr/WeakPtr if that is the case. I'm not completely sure about this without trying it myself, but I am thinking that might be an issue.

[gorlak]

complete.