Hide ToSystemType conversions behind an interface.

[ewoutkramer]

The following datatypes (to which Instant should be added) have methods to convert their values to a CQL type.

*Code
*CodeableConcept
*Code
*Coding
*Date
*FhirDateTime
*Time
*NEW Instant

There is no way to polymorphically call these, which would be really useful in the implementation of IScopedNode.Value. So, create an interface with a single ToSystemType() method, hide these methods behind an explicit implementation and adapt the implementation of IScopedNode.Value to use these methods instead.