Note: this contains two _temporary_ changes which can be reverted
once the Arena vs no-Arena distinction in API wrappers has been removed.
Both nlua_push_Object and object_to_vim_take_luaref() has been changed
to take the object argument as a pointer. This is not going to be
necessary once these are only used with arena (or not at all) allocated
Objects.
The object_to_vim() variant which leaves luaref untouched might need to
stay for a little longer.
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