2781 Work on ObjectValue

[ewoutkramer]

Fixes #2781.

This PR contains several improvements to how ObjectValue and validation are handled on primitives:

• ObjectValue now always aligns with the json literals, so the ObjectValue is now string for Instance (not DateTimeOffset), and string for Base64Binary (not byte[]).
• It has delayed parsing from ObjectValue->Value and delayed serialization from Value->ObjectValue for all types.
• It unifies the validation of the ObjectValue across all primitives: all operations/properties on primitives that access Value will make sure the data is valid, and throw CodedValidationExceptions otherwise.
• Because the primitives now handle validation of ObjectValue, the parsers don't have to do it anymore, and they have been simplified to mostly just copy the raw value on the line to the ObjectValue, and let the new Primitive validation handle the rest.

⚠️ Make sure to also review and pull FirelyTeam/fhir-codegen#52