* add notice to files being removed
* add gin version of function_template.h
* rename callback.h to avoid confliction
* add gin version of callback_converter
* add gin converter for OnceCallback
* remove callback_converter_gin_adapter.h
* remove gin_util.h and gin_utils.h
* fix lint warning
* add helper for setting methods
* move Destroyable utilities out of native_mate
* do not set "destroy" in ObjectTemplateBuilder
* remove ObjectTemplateBuilder::MakeDestroyable
* do not pollute gin namespace
* add more comments
* remove hack of Arguments