Instance registry

[uvlad7]

It's not a some specific bug, actually, I just want to discuss instance registry problems highlighted in the docs.

There could be a window where a Ruby object is marked for deletion but the underlying C++ object is returned back to Ruby

I feel like it's possible to reuse ObjectSpace._id2ref logic here, and take a look on how gc frees T_DATA. Or maybe use weakmap.

Ruby instance may be freed on the C++ side

Is it possible to enable the registry only for Ruby-managed instances? Maybe use Ruby's xmalloc somehow?