Add the CQL system types as POCOs through DynamicPrimitive

[Kasdejong]

While refactoring the FP engine against POCOs, we encountered issues when handling constants, since these are usually considered to be CQL types as opposed to FHIR types. Luckily, we now have DynamicPrimitive to represent them!

Note: We should confine these to the FP engine somehow. Maybe even by making them internal.