Include fragment in `mnat_auth_auth`
This gives the following new lemma:
Lemma mnat_auth_included q n : mnat_auth_frag n ≼ mnat_auth_auth q n.
Also reorganize the files slightly so that validity lemmas are grouped together.
This follows the approach I suggested for the monotone bijections RA here. It shifts more of the work to the RA-level library, making it easier for users of Iris to implement variants of the logic-level library.
PS: Why is this library called
mnat_auth and not