Merge pull request #4787 from shuujii/optimize-str_subseq-with-MRB_UTF8_STRING-to-ASCII-only-string

Optimize `str_subseq` with `MRB_UTF8_STRING` to ASCII only string

src/string.c

@@ -603,7 +603,7 @@ str_range_to_bytes(mrb_value str, mrb_int *pos, mrb_int *len)
 static inline mrb_value
 str_subseq(mrb_state *mrb, mrb_value str, mrb_int beg, mrb_int len)
 {
-  str_range_to_bytes(str, &beg, &len);
+  if (!RSTR_ASCII_P(mrb_str_ptr(str))) str_range_to_bytes(str, &beg, &len);
   return mrb_str_byte_subseq(mrb, str, beg, len);
 }
 #else