v8.3: Fixing bug #3169 and avoiding anomaly in bug #2885 (vm_compute not
supporting metas/evars). Fix of #3169 is by calling pretyping retyper
rather than the non evar-aware kernel type-checker (since argument of
vm_compute is supposed to be already typable).
v8.3: Fixing bug #3228 (fixing precedence of ltac variables over
variables in env).