high: parse: Support id-ref in nvpairs (fate#316118)

Support <nvpair id-ref="target"/>
and <nvpair id-ref="target" name="name"/>

To explicitly set the id of an nvpair:
 $<id>:<name>=<value>
To refer to an nvpair:
 @<id>
To refer and rename:
 @<id>:<name>