fhirpath expressions

[barabo]

If a FHIR Resource defines a fhirpath expression, that expression should inform any logic defined in the module/resource/*/* code. This module should be as familiar as possible for fhirpath users, even if a full implementation of that spec is not possible.

As Resource facades are written, a 'lint-like' operation could emit a warning if a fhirpath expression defined for a FHIR Resource is not implemented for the facade.