Improve execution output

[brynrhodes]

Right now the execution just renders to text, would be nice to have a more formal serialization

Consider CQL Serialization, proposed here:

https://github.com/cqframework/clinical_quality_language/wiki/CQL-Serialization

Consider FHIR-based serialization, specified here:

https://hl7.org/fhir/uv/cql/conformance.html#fhir-type-mapping