Consolidate CQL datatypes

[ewoutkramer]

Both our FHIR and CQL SDK have a set of CQL datatypes, that are covering exactly the same ground. They were created for the same purpose, but, back then, by separate teams. It's been our goal for a while to pick a moment when we are going to consolidate those types in a shared assembly.

Is 6.0 the time? There is certainly a good reason to converge, not only do we have duplicate code, but we also do unnecessary interconversions from FhirDate <-> our CqlDate <-> Cql SDK's CqlDate. Also, we could probably improve caching or avoid parsing if we make a single set of types.

I don't know whether we have time for this, as it also requires coordination with the CQL SDK team, but I will put this action here as a reminder in case we have time left, or have shifting priorities.