Fix extraction taking a type in the wrong environment.
Fix restriction of universe contexts to not forget about potentially useful
constraints.
- Fix treatment of global universe constraints which should be passed along
in the Evd of proofs (Evd.from_env).
- Allow to set the Store.t value of new evars, e.g. to set constraint evars as
unresolvable in rewrite.ml.
- Fix a HUGE performance problem in the processing of constraints, which was remerging
all the previous constraints with the ambient global universes at each new constraint addition.
Performance is now back to (or better than) normal.