Create standard function library

[ewoutkramer]

In addition to the operators with specific syntax, there is a whole list of "normal" standard functions which are represented using their own ELM nodes. We need to create a "System" library with all these functions consisting of:

• https://build.fhir.org/ig/HL7/cql/09-b-cqlreference.html#logical-operators-3 (and further)
• https://build.fhir.org/ig/HL7/cql/16-i-fhirpathtranslation.html
• https://build.fhir.org/ig/HL7/cql/06-translationsemantics.html

Since the System library (much like the System types) is always "included" we do need to add some magic to make sure these are always in scope as well.