* Fix issues with uint64 enums
- hide the implementation of enums from code generators
- fix uint64 the issue in the cpp-generator
- fix#5108
- new tests
- enums with bit_flags attribute should be unsigned
* Refine objectives of EnumDef's FindByValue and ReverseLookup methods
- move EnumDef::ReverseLookup implementation to idl_parser.cpp
- fix typos
* Make the IsUInt64 method private