ruby: create an actual wrapper struct

Currently Ruby data points directly to a notmuch object (e.g.
notmuch_database_t), since we don't need any extra data that is fine.

However, in the next commit we will need extra data, therefore we create
a new struct notmuch_rb_object_t wrapper which contains nothing but a
pointer to the current pointer (e.g. notmuch_database_t).

This struct is tied to the Ruby object, and is freed when the Ruby
object is freed by the garbage collector.

We do nothing with this wrapper, so no functionality should be changed.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>