move mrb_proc_new_cfunc_with_env() to the core
refactor valid instance variable name check
implement attr_reader and attr_writer in C; use cfunc closure to speed-up
Refactor obj_is_instance_of() in kernel.c
Refactor mrb_obj_is_kind_of_m() in kernel.c
Merge pull request #2580 from suzukaze/refactor-kernel.c

Refactor kernel.c