general: add support for declaring enums as struct members

the enum types are resolved to the wiretypes for the benefit of c-loxi.
py_loxi: support/resolve enum members of structs
openflow_input: declare of_flowmod.flags as enum ofp_flow_mod_flags

others will follow
java_gen: support enum types, sets of enum types
loxi_ir: add convience method is_bitmask
test_data for java_gen(of13): use new API