Add mapping from ARC choices to FHIR standardised clinical terminology

[abhidg]

Currently ARC questions have SNOMED codes, but the choices do not. For simple choices (Yes, No), standardised FHIR terms can be used, but for everything else (such as Smoking: Yes / Former / Never) or case status, there should be a mapping from (question, choice) pairs to FHIR terminology.

Example of dengue data mapping, which corresponds to ARC questions
https://docs.google.com/spreadsheets/d/15nQwXBIKnXF9lRHbVVdfFxOMGfLqhOfxZ7GkjSu_Kcs/edit?gid=462215516#gid=462215516

Mapping file specification
https://fhirflat.readthedocs.io/en/latest/spec/mapping.html

This is required by globaldothealth/fhirflat#60