Merge pull request #1634 from cremno/use-mrb_fixnum_p

tiny word boxing optimization

src/string.c

@@ -1243,7 +1243,7 @@ mrb_str_include(mrb_state *mrb, mrb_value self)
   mrb_bool include_p;
 
   mrb_get_args(mrb, "o", &str2);
-  if (mrb_type(str2) == MRB_TT_FIXNUM) {
+  if (mrb_fixnum_p(str2)) {
     include_p = (memchr(RSTRING_PTR(self), mrb_fixnum(str2), RSTRING_LEN(self)) != NULL);
   }
   else {