wireshark_gen: move enum table out of the template
wireshark_gen: generate enum usage from IR
wireshark_gen: resolve enum wire types during generation

Instead of indirecting at runtime. This fixes reading all enums.
wireshark_gen: fix exception on empty match

The peek_all() call fails if the buffer is empty.
wireshark_gen: add reader for ipv6
Merge into master from pull request #106:
Wireshark enum fixes (https://github.com/floodlight/loxigen/pull/106)