Tentative: share grant table code with Mirage

This is the new code refresh from ocaml-xen-lowlevel-libs. I think 2 changes will be needed to client code:
* ocamlfind package renamed to xen-gnt
* "standardised" spelling of "writable" (I googed for it, looked on the email threads in horror and am hoping we can just do it)

If you can point me at any code which will need updating, I'm happy to do the update myself!