VST release 2.2

Release version 2.1

Some minor tweaks to the Hint Rewrite databases "norm" and "gather_prop"

that improve normalize and entailer; improves issue PrincetonUniversity#142 as well.

Merge branch 'master' of github.com:PrincetonUniversity/VST

VST release 1.8

Release version 1.7 of VST

Release version 1.6

Branch new_compcert (for porting to CompCert 2.5) is no longer an act…

…ive branch

Branch compcert2.6 is no longer an active branch

Branch compcert2.4 is no longer an active branch