Support referencing container resources from contained resources

[jafeltra]

When a contained resource references its own container, the reference it uses is just #. SUSHI should allow specifying # from within the Reference() keyword.

See FHIR documentation here: https://www.hl7.org/fhir/references.html#contained. Relevant snippet is:

For a resource that references the container, the reference is "#", like this:

<Patient xmlns="http://hl7.org/fhir">
 <id value="something"/>
 <contained>
   <Provenance>
     <!-- no id necessary (though still allowed) -->
     <target>
       <reference value="#"/>
     </target>
   </Provenance>
 </contained>
 <!-- other attributes -->
</Patient>

An example of what the FSH for this could look like is here: FSH Online Link.

This was brought up in Zulip here: https://chat.fhir.org/#narrow/channel/215610-shorthand/topic/Contained.20resource.20can't.20reference.20back.20to.20containing/near/486328555