IntOrString for use in JSON/YAML

Specifying an API type as IntOrString will allow JSON and YAML to accept
either ints or strings with the same name.  For example, port names or
numbers.
Yeah, I wasn't sure what to recommend there. It's impossible now, but if you ever extended IntstrKind to have a third kind and forgot to update the yaml conversion code, we wouldn't notice during compilation and we'd start seeing programs crashing with panics.