(piqi) Uncomment previously commented out section
(piqi) Allow referencing *-input|-output|-error definitions

There were a couple of problems with the artificial limitation prohibiting this:
- sometimes, evolving shemas actually need to do that in order to avoid
  modification to the original spec
- there is no difference between definitions defined in-line a part of function
  parameters and top-level definitions: nested defs can be transformed into
  equivalent top-level defs and the other way around (the latter is pretty
  useless through)
- there was an actual bug that prevented referencing legitimate top-level defs
  named *-input|*-output|*-error if there was at least one function that
  referenced them