R4: Update snapshot generator for type slicing

[wmrutten]

Update the snapshot generator implementation according to changes discussed with Grahame at DevDays US in Redmond.

Specifically, type slice in snapshot should always include slicing component. Slicing component may be omitted from the differential (implicit/redundant for type slice); in that case, the generator should emit a default type slicing component to the snapshot.

WARNING! R4 API packages did NOT process type slices correctly. Once this has been fixed, customers will have to re-generate some existing snapshots.

[wmrutten]

TODO: Implement Grahame's Snapshot unit tests

https://github.com/hapifhir/org.hl7.fhir.core/blob/master/org.hl7.fhir.r5/src/test/resources/snapshot-generation/manifest.xml

[wmrutten]

Test results are published here:
https://gist.github.com/wmrutten/acc5e24601eb74df582ff39b21f41efc

[ewoutkramer]

Fixed for R4 in #1088