Add `userdata` to tastudio greenzone states

The userdata lua library currently only works with regular savestates but not tastudio greenzone (which uses memory savestates iirc?), which makes it problematic to restore lua values from past tastudio frames for things that can't be calculated on the fly for every given frame.

I have a counter that increments on a memory hook, and I need to store it in savestates in some way so I could go back without it infinitely incrementing on every advance and rewind, and I need to it also go back and show me values it had back then.

I think memory savestates altogether should include userdata whenever it's there (which is rare anyway, so shouldn't mess up anyone's workflow).

Start by tracing how the userdata Lua library is serialized for regular savestates and how Tastudio greenzone memory savestates are created or restored. Done means userdata values, including values changed by memory hooks, are restored when moving backward and forward through Tastudio greenzone frames without disrupting existing savestate workflows.