Log an error when an instance assigns multiple types from the same choice element

[cmoesel]

FHIR choice elements (e.g., value[x]) are always 0..1 or 1..1. As such, there can never be more than one value for a given choice element. Once an author has assigned a value for one of the types, they cannot also assign a value to a different type.

For example, the following should result in an error and the second value[x] assignment (valueQuantity = 5 'cm') should be ignored:

Instance: MyTwoValuedObservation
InstanceOf: Observation
* status = #final
* code = #foo
* valueBoolean = true
* valueQuantity = 5 'cm'

Currently, SUSHI does not emit and error and exports both values into the instance JSON.

See: https://chat.fhir.org/#narrow/channel/215610-shorthand/topic/instance.20with.20extension.20incl.2E.20multiple.20value.5Bx.5D/near/477466665