Fix processing of slices on choice elements

[cmoesel]

Description: When manually creating slices on choice elements (e.g., Observation.value[x]), SUSHI runs into a few issues:

1. If you constrain a slice by type (using only) and then constrain another by type (also using only), in some cases SUSHI emits an error like this: Type constraint on Observation.value[x] makes rules in ObservationWithValueAttachment obsolete for choices: valueString.
2. If you attempt to traverse into a sub-element of a slice, in some cases SUSHI emits an error like this: No element found at path value[x][valueAttachment].extension for ContainsRule in ObservationWithValueAttachment, skipping rule. (NOTE: error depends on type of rule applied to the path).

These issues were discovered and discussed on Zulip: #shorthand > Slicing and extending with a xver extension @ 💬.

This PR resolves both issues by improving handling for the special case of slices on choice elements.

Testing Instructions: The unit tests show the issue, but you may also test manually using this XVerObsValueAttachment.zip project based on the Zulip thread use case. First run the release version of SUSHI to confirm the errors are being logged. Then run this PR branch (using ts-node) to confirm the errors go away and the resulting StructureDefinition looks correct.

Related Issue: See https://chat.fhir.org/#narrow/channel/215610-shorthand/topic/Slicing.20and.20extending.20with.20a.20xver.20extension/near/604137635.