for example, loading a library which returns a table of functions would
fail with an inexplicable (to the user) error about serialization. This
is now fixed. We no longer use a single result type, instead there are
void results and normal results, and the former simply never has to
worry about the result.
This was likely causing even more issues; if the `sol::object` was
populated before, it hitting the dtor in another path would, again, like
we've seen before, corrupt the lua stack.
this massively improves thread safety and cleanly serializes accesses
into the lua engine's result objects where accesses before were
extremely unsafe and could access a corrupt/invalid stack.
this fixes various obscure crashes related to accessing results,
without changing any observable behavior.