Implement 'iso proto proto-id'

* src/pf/expand.lua: Implement lowering of iso protocol.
* src/pf/parse.lua: Support parsing of iso protocols as single words.
* doc/Makefile: Modify Makefile to add new example.
* doc/iso-proto-clnp.md: Add example "iso proto clnp".
let's golf this a bit lower

return has_iso_protocol(assert(iso_protos[expr[2]], "Invalid ISO protocol"))