R5 Support

[mikix]

fhirclient currently only handles R4 - but it would be nice if it added R5 support too.

Some things I know would need to happen:

• fhir-parser needs to complete its R5 support (at time of writing, it can generate R5, but tests don't fully pass)
• We should probably fix No good way to use multiple versions of the library in a single project? #49 so that we can support both R4 and R5 at the same time.

This would be a good opportunity to bump the major version and thus introduce other mildly breaking changes if we wanted. For example, I know there are a couple comments with the tag R4-QUIRK that could probably use tweaking once we make the jump.