Permit Constraints with Void Context

[DavePearce]

At the moment, when a vanishing constraint has the void context then a failure is raised. However, it is possible for valid situations to arise where the void context arises. Specifically, an example like this:

(defconstraint stamp-increment-sanity-check ()
  (begin                                                                      
    (vanishes! 0)                                                                
    (debug (any! (will-remain-constant! STAMP) (will-inc! STAMP 1)))))

Whilst this is a somewhat undesirable example, its still useful to support it at this time.