Add rigor to structured terminology

[mbrush]

Originally posted by @rhdolin in #234 (comment):

Many attributes of type 'string' should be migrated to type 'coding' (e.g. ConceptMapping.relation), with provision of value sets (e.g. code lists) and a registry of known code systems where possible. I know we've talked about this and it's a recognized issue that is on the backburner, so just thought I'd log it here. I'd suggest looking at the FHIR terminology approach, which has evolved over many years in HL7.

[mbrush]

Agree with this idea, and the notion that this will likely come in future versions of the spec. But is out of scope for the ballot / 1.0 release.