java_gen: update pre-written types

allow for instances of U8,U32, U64
add Writeable interface
java_gen: reworked type system, cleaned up ChannelUtils
java_gen: enable generation for all messages, lots of related fixes
openflow_input: add forgotten discriminators
test_data: add java section for of13/packet_in
Merge branch 'master' of github.com:andi-bigswitch/loxigen