use MRB_STRINGIZE

It's defined in mruby/version.h which gets included in mruby.h, so it's
safe to use.

src/vm.c

@@ -52,9 +52,6 @@ The value below allows about 60000 recursive calls in the simplest case. */
 # define DEBUG(x)
 #endif
 
-#define TO_STR(x) TO_STR_(x)
-#define TO_STR_(x) #x
-
 #define ARENA_RESTORE(mrb,ai) (mrb)->arena_idx = (ai)
 
 static inline void
@@ -160,7 +157,7 @@ stack_extend_alloc(mrb_state *mrb, int room, int keep)
      to prevent infinite recursion. However, do this only after resizing the stack, so mrb_raise has stack space to work with. */
   if (size > MRB_STACK_MAX) {
     init_new_stack_space(mrb, room, keep);
-    mrb_raise(mrb, E_SYSSTACK_ERROR, "stack level too deep. (limit=" TO_STR(MRB_STACK_MAX) ")");
+    mrb_raise(mrb, E_SYSSTACK_ERROR, "stack level too deep. (limit=" MRB_STRINGIZE(MRB_STACK_MAX) ")");
   }
 }
 
@@ -301,7 +298,7 @@ mrb_funcall(mrb_state *mrb, mrb_value self, const char *name, mrb_int argc, ...)
   mrb_sym mid = mrb_intern_cstr(mrb, name);
 
   if (argc > MRB_FUNCALL_ARGC_MAX) {
-    mrb_raise(mrb, E_ARGUMENT_ERROR, "Too long arguments. (limit=" TO_STR(MRB_FUNCALL_ARGC_MAX) ")");
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "Too long arguments. (limit=" MRB_STRINGIZE(MRB_FUNCALL_ARGC_MAX) ")");
   }
 
   va_start(ap, argc);