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

last version of VST compatible with CompCert 2.5

Version 1.5.3. Best version if you want expr_rel. (before new data_at)