Fix issues with uint64 enums (#5265)

* 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

src/idl_gen_lobster.cpp

@@ -263,7 +263,7 @@ class LobsterGenerator : public BaseGenerator {
       auto &ev = **it;
       GenComment(ev.doc_comment, code_ptr, nullptr, "    ");
       code += "    " + enum_def.name + "_" + NormalizedName(ev) + " = " +
-              NumToString(ev.value);
+              enum_def.ToString(ev);
       if (it + 1 != enum_def.Vals().end()) code += ",";
       code += "\n";
     }