To help people debugging cyclic references, we now trace the "reason"
a cyclic reference got added, so that we can present the conflicting
references instead of just presenting an error. Fix the order of
the error message, which was the wrong way around.

Clean up references a little:

- Split out `Reference` and `CfnReference`, which got conflated in an
  undesirable way. `Reference` is now officially the base class for all
  references, and `CfnReference` is only one implementation of it for
  CloudFormation references.
- Make 'scope' required for references, the only place where it was
  potentially empty was for CFN Pseudo Parameters, refactored those to
  not use classes anymore (because there's no need to).
- Get rid of 'Ref', the class wasn't being very useful.
- Make a dependency Construct => Stack lazy (it was conflicting at
  load time with the Stack => Construct dependency which is more
  important).