loci: generate wire type parsers from the IR

Every virtual class gets a function that reads the discriminator and returns an
object id. If the subclass is a virtual class then the parsing recurses.
loci: remove handwritten wire_object_id_get functions
loci: remove more dead code
loci: switch message parsing to of_header_wire_object_id_get
loci: add missing test for of_object_new_from_message

Also fix embarassing typos in test_of_object_new_from_message_preallocated.
Merge remote-tracking branch 'origin/master' into loci-parser

Conflicts:
	c_gen/c_type_maps.py
	c_gen/templates/of_type_maps.c
Merge into master from pull request #216:
Generate LOCI wire type parsers from the IR (https://github.com/floodlight/loxigen/pull/216)