loxigen: sort ofclasses by name
move OXM definitions into the input files

This makes the rest of the code simpler, and makes it easier to add new OXM
types in extensions.
move OXM wire types into type_maps.py
parser: tag data members

It's cleaner to differentiate between the different types of member AST by
using the first element as a tag.
frontend: support specifying type member values in the input file
openflow_input: add type values