Getting id and resourceType of reference ( getId, getResourceType )

[niquola]

We need to abstract how references are stored in a database. Most implementations do pre-parse references and store resourceType and id separately (in our spec on the storage layer, we suggest using {type, sof_id} elements for that:

subject: {reference: 'Patient/pt-1'}
// translated into
subject: {type: 'Patient', sof_id: 'pt-1'}

To support this abstraction, we need functions in fhirpath to get id and type:

subject.getId()
subject.getId('Patient')
subject.getId('Group')
subject.getResourceType()