java_gen: delete HashFunc interface

only used internally in floodlight and not needed here
java_gen/U* types: change toString to hex

We mostly use the U* classes to represent non-numeric data.
Hex strings seem like a more useful representation.
OFErrorMsg: add OFErrorCauseData wrapper for nicer toStrings BSC-5349
Merge into master from pull request #263:
OFErrorMsg: add OFErrorCauseData wrapper for nicer toStrings BSC-5349 (https://github.com/floodlight/loxigen/pull/263)