copy_values is not a big if-then-else anymore.
Before this change, whenever we added a new kind of userdata, we would have to update copy_values.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Lua API is an integral part of Lean. It does *not* have the same status
of external APIs (e.g., Python) we will add in the future.
We will reserve the directory bindings for external APIs for using Lean
as a library.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>