Expose RawLua

[cheesycod]

This is needed for implementing custom thread resumption and converting between stack and LuaValue's etc.

[khvzak]

I don't think it's a good idea to expose any private structs or methods, this is out of mlua goals and scope.
Some of private methods like IntoLua::push_into_stack may be unintentionally available due to language limitations but they are not designed for public use.

Also, in this form RawLua will be useless because all their methods are marked as pub(crate).