Allow structs to be parsed in JSON with out of order fields.

Also simplified the code and made it faster.

Change-Id: I1d83b1165a4a9a4380d1bfb5538769c012d2d367
Tested: on Linux.

tests/monster_test_generated.h

@@ -167,6 +167,7 @@ inline flatbuffers::Offset<Stat> CreateStat(flatbuffers::FlatBufferBuilder &_fbb
   return builder_.Finish();
 }
 
+/// an example documentation comment: monster object
 struct Monster FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   const Vec3 *pos() const { return GetStruct<const Vec3 *>(4); }
   Vec3 *mutable_pos() { return GetStruct<Vec3 *>(4); }