loci: automatically derive show emitters

Previously you needed to add a macro for each (member type, member name). This
was a common stumbling block for people contributing to the input files. Now
you only need to modify LOCI's show code if you want to customize how the field
is displayed (hex, for example).
Merge into master from pull request #327:
loci: automatically derive show emitters (https://github.com/floodlight/loxigen/pull/327)