* Main change: make the error message generated by mkFunTysMsg more
  accurate by changing "value arguments" to "visible arguments".

* Refactor: define a new type synonym VisArity and use it instead of
  Arity in a few places.

It might be the case that there other places in the compiler that should
talk about visible arguments rather than value arguments, but I haven't
tried to find them all, focusing only on the error message reported in
the ticket.