Set up a basic nominal set library.

So far, there is no interaction with the rest of the developpement.
Freshness in nominal sets.
Additions to the nominal set library.
Define nominal instances for the datastructures in shape graphs.

Includes a wrapper around Compcert's tries which are ensured to be in canonical form (in particular extensional equality implies Leibniz equality). This allows the definition of an action on said tries without having to change the action to handle setoid equalities.
Proofs that functions are equivariant to replace the ad-hoc "central" lemmas.
README: add info on the nominal sets.
README: add link to the arxiv paper on refinement & abstract-interpretation