use regular macros instead of compatibility macros

According to mruby/include/mruby.h,

 /* compatibility macros; will be removed */
 #define ARGS_REQ(n)         MRB_ARGS_REQ(n)
 #define ARGS_OPT(n)         MRB_ARGS_OPT(n)
 #define ARGS_REST()         MRB_ARGS_REST()
 #define ARGS_POST(n)        MRB_ARGS_POST()
 #define ARGS_KEY(n1,n2)     MRB_ARGS_KEY(n1,n2)
 #define ARGS_BLOCK()        MRB_ARGS_BLOCK()
 #define ARGS_ANY()          MRB_ARGS_ANY()
 #define ARGS_NONE()         MRB_ARGS_NONE()