Now, simply remove the userdata table immediately before dropping the userdata.
This does two things, it prevents __gc from double dropping the userdata, and
after the first call to __gc, it prevents the userdata from being identified as
any particular userdata type, so it cannot be misused after being finalized.
This change thus removes the userdata invalidation error, and simplifies a lot
of userdata handling code.
It also fixes a panic bug. Because there is no predictable order for
finalizers, it is possible to run a userdata finalizer that does not resurrect
itself before a lua table finalizer that accesses that userdata, and this means
that there were several asserts that were possible to trigger in normal Lua code
in util.rs related to `WrappedError`.
Now, finalized userdata is simply a userdata with no methods, so any use of
finalized userdata becomes a normal script runtime error (though, with a
potentially confusing error message). As a future improvement, we could set
a metatable on finalized userdata that provides a better error message.
Also make sure that panic messages clearly state that they are internal errors,
so people report them as a bug. Since the only panics left are all internal
errors, just move the internal error message into the panic / assert macros.
First, make sure that `add_methods` cannot trigger another userdata registry
insert, causing an unintended panic. Second, remove `RefCell` surrounding
userdata hashmap, as this change makes it no longer needed. Third, add a
`RefCell` around `Callback` because FnMut means that callbacks cannot recurse
into themselves, and panic appropriately when this happens. This should
eventually be turned into an error.
kyren