Minimize memory sharing in UCDD library

[magoorden]

cdd_info(state.root)-->type == TYPE_BDD

I wonder how did it work before? :-/
This is such a mess of memory sharing, which needs to be rewritten with move semantics instead.

Originally posted by @mikucionisaau in #31 (comment)

[mikucionisaau]

That comment was due to the changes in that PR where root is private and handle() was a new public method to access it.
Later I found that .root worked due to the fact that the owner structure has many function friends, which enabled access to private fields in many places.
I think the intention was to have a tight control over which functions can access the internals ('private' members), so perhaps it is worthwhile continueing it like that.