Consider variables with conlike unfoldings interesting

In this expression:

  let x = f (g e1) in e2

the simplifier will inline f if it thinks that (g e1) is an interesting
argument. Until now, this was essentially the case if g was a data constructor
- we'd inline f in the hope that it will inspect and hence eliminate the
constructor application. This patch extends this mechanism to CONLIKE
functions. We consider (g e1) interesting if g is CONLIKE and inline f in the
hope that this will allow rewrite rules to match.