Hide all raw pointers manipulation by users behind an opt-in feature flag

This should reduce the risks of unsafe use with regards to aliasing. Users can still manipulate the raw pointers directly (btw. something the original crate didn't allow outside of ne or two objects where it needed funtions for itself, so we are not reducing int he default configuration the scope of user actions when compared to nftnl-rs, but adding - from the POV of an ex-nftnl-rs user - new unsafe features hidden behing a cargo feature flag).

As another importance side-note, this PR also switches Rc instead of Arc and drop the Send/Sync to increase misus-resistance in multithreaded environemnts.

What do you think ?