Note [CallStack and ExceptionContext hack]

It isn't really right that we treat CallStack and ExceptionContext dictionaries
as canonical, in the sense of Note [Coherence and specialisation: overview].
They definitely are not!

But if we use EvNonCanonical here we get lots of
    nospec (error @Int) dict  string
(since `error` takes a HasCallStack dict), and that isn't bottoming  (at least not
without extra work)  So, hackily, we just say that HasCallStack and ExceptionContext
are canonical, even though they aren't really.