Fix exprIsConApp_maybe

In this commit
   commit 7833cf40
   Date:   Thu Jan 24 17:58:50 2019 +0100
      Look through newtype wrappers (Trac #16254)

we made exprIsConApp_maybe quite a bit cleverer.  But I had not paid
enough attention to keeping exactly the correct substitution and
in-scope set, which led to Trac #16348.

There were several buglets (like applying the substitution twice in
exprIsConApp_maybe, but the proximate source of the bug was that we were
calling addNewInScopeIds, which deleted things from the substitution as
well as adding them to the in-scope set.  That's usually right, but not
here!

This was quite tricky to track down.  But it is nicer now.