Instantiate: fill in more of the cases in eval
Instantiate: fill in more of the cases in eval

Hopefully this is all of them that we'll ever need. Cases not included
are those in grepTopApps, ones that don't have Exprs in them, constants,
gradual type predicate `PGrad`, quantifiers.

fixes ucsd-progsys/liquidhaskell#997

ucsd-progsys/liquid-fixpoint