Investigate userdata safety in synced

AFAIK userdata types were/are 100% forbidden in synced luarules. I assume this is because of desync, but I don't remember this being said explicitly anywhere.

Userdata allows neat interfaces so it would be good to investigate how dangerous they really are. Test some obvious scenarios:
* what if you do `table[userdata] = bla`, and then `table.sort` or `for ... in pairs`? Is the order deterministic?
* can you get some sort of nondeterministic, address-based value from a userdata via things like `string.format` or `tostring`?
* what about more esoteric things like GC hooks, metamethods, cleanup time/order when used as a key in a table set to use weak keys, or interactions with weird interfaces like the lua registry or Recoil cross-env calls (`SYNCED.bla`, `local userdata_from_luaui = Script.LuaUI.Bla`, etc)?

#2343 adds userdata to synced, so it's a good testbed. (It's fine to merge 2343 even if this investigation isn't done, gamedevs will just have to remember not to do anything stupid with the interface.)

Use #2343's synced userdata support as the testbed and exercise the listed table, formatting, GC, metamethod, weak-key, registry, and Recoil cross-environment scenarios. Record whether each behavior is deterministic and safe for synced luarules, then document the resulting constraints for gamedevs.

Written by the indexing model from the issue text.