Implement FhirPath subtraction operations for dateTime and date in the default symbol table

[almostchristian]

FhirPath subtraction for dateTime and date are currently not included in the standard functions, but can be simulated by changing the expression to use binary.+ and unary.- which is cumbersome.

Fails:
Patient.birthDate - 100 years
Exception:
Invocation of operator '-' failed: Unable to cast object of type 'Hl7.Fhir.ElementModel.Types.Date' to type 'Hl7.Fhir.ElementModel.Types.Quantity'.

Works:
Patient.birthDate + (-100 years)

We are able to use the desired expression by adding the operations to the symbol table which I think should be included by default.

FHIRPath date/time subtraction

FhirPathCompiler.DefaultSymbolTable.Add("binary.-", (object f, P.DateTime a, P.Quantity b) => a + new P.Quantity(-b.Value, b.Unit), doNullProp: true);
FhirPathCompiler.DefaultSymbolTable.Add("binary.-", (object f, P.Date a, P.Quantity b) => a + new P.Quantity(-b.Value, b.Unit), doNullProp: true);