Add "handle" smart pointer

[Kazuo256]

There are a few cases in UGDK where a manager-thing holds the ownership of a pointer, but lends out "weak references" to it.

In some places we use shared/weak pointers. In others, we use raw pointers.

I suggest we implement a "handle pointer" sort of thing, which can access the originally owned pointer as long as it is valid. See 15.5.3 from Game Engine Architectures.