[Feature] Further optimization to the hot path

[Feature] Further optimization to the hot path

We check userdata very frequently, so the idea here is the following:

- Store the address of classname (converted to int) in the metatable at index 1
- When we need to check some udata, we can just compare the static address with
  the integer stored in metatable
- This avoid quite an expensive `lua_rawequal` call for two tables as we know that
  our classes are quite static