Add .references() fluent functions as proposed in Cooking with CQL 82

[brynrhodes]

Consider adding .references() fluent functions as proposed in Cooking with CQL Session 82:

define fluent function references(reference FHIR.Reference, resource FHIR.Resource):
  resource.id = Last(Split(reference.reference, '/'))  

define fluent function references(references List<FHIR.Reference>, resource FHIR.Resource):
  exists (references R where R.references(resource))

https://github.com/cqframework/CQL-Formatting-and-Usage-Wiki/blob/master/Source/Cooking%20With%20CQL/82/TestHIVAssay.md#fhir-references