loxi_ir: automatically generate action/instruction ID classes

These are used in the table features multipart messages.
py_gen: move ID classes into their own modules
loci: use ID classes from the IR

Instead of building them itself.
java_gen: support action and instruction IDs
Merge into master from pull request #151:
automatically generate action/instruction ID classes (https://github.com/floodlight/loxigen/pull/151)