It now turns setoid equalities into Leibniz equalities when possible,
and substitutes those.

In principle, we could now un-seal heap_mapsto, saved_prop_own etc., and mark them as "Typeclass Opaque", and ecancel would still work just as fast as it does now.
Thanks to Matthieu for pointing me to this unify feature.

Conflicts:
	prelude/tactics.v

This replaces f_equiv and solve_proper with our own, hopefully better, versions

The performance gain seems neglectable, unfortunatelly...

* Use sig instead of sigT: the proof is a Prop after all
* Tweak implicit arguments
* Shorten proof of sigma

It now traverses terms at most once, whereas the setoid_rewrite
approach was travering terms many times. Also, the tactic can now
be extended by defining type class instances.

Conflicts:
	barrier/barrier.v