Integrate FhirPath static analysis #3313
Description
@brianpos has written a static analyzer for the FhirPath engine: https://github.com/brianpos/Hl7.Fhir.FhirPath.Validator/blob/master/src/Hl7.Fhir.Base.FhirPath.Validator
This is written against SDK5, but I propose adding this to SDK6. While doing this, we can improve the code a bit, since Brian had to work around the fact that Expression had some read-only members that we might want to change in SKD6. Since the deadline for SDK6 changes has passed, this might even mean SDK7. We could also decide to treat the static analysis result as annotations, this would require fewer changes to the Expression class.
Anyway, steps would include
- Add nullability annotations to the Expression classes
- Obsolete the current ExpressionType property (which is using the defunct CQL typing classes that we will not use anymore)
- Change the NodeProp etc. classes in this PR into annotations
- Change the code the annotate the expression nodes.