loci: generate match tables from the IR

This removes most of the manual work from adding a new OXM. As a side effect,
it fixes locitest so it doesn't try to test OpenFlow 1.3 OXMs with an OF 1.2
message.
loci: address review comments
Merge into master from pull request #274:
loci: generate match tables from the IR (https://github.com/floodlight/loxigen/pull/274)