Validator Changes

Fix expression for con-3 properly (fix validation problem on some condition resources)
Fix FHIRPath bug using wrong type on simple elements when checking FHIRPath types
FHIRPath: Allow _ in constant names (per FHIRPath spec)
Fix value set rendering creating wrong references
Fix bug processing value set includes / excludes that are just value sets (no system value)
Alter processing of unknown code systems per discussion at ,https://chat.fhir.org/#narrow/stream/179252-IG-creation/topic/Don't.20error.20when.20you.20can't.20find.20code.20system and implement unknown-codesystems-cause-errors
Improve message for when elements are out of order in profile differentials

Other code changes

fix problem where profile rendering had spurious 'slices for' nodes everywhere
Update SQL-On-FHIR implementation for latest cases, and clone test cases to general test care repository
Fix problem generating value set spreadsheets
fix concurrent modification error processing language translations
Check for null fetcher processing ConceptMaps (#1728)