docs: public final CharSlice substring(int start, int end) {
          if (start < 0 || end > len || end < -len) {
              throw new IllegalArgumentException(
                      String.format("[%d,%d] of slice length %d is not valid.", start, end, len));
          }
          int l = end < 0 ? (len - start) + end : end - start;
          if (l < 0 || l > (len - start)) {
              throw new IllegalArgumentException(
                      String.format("[%d,%d] of slice length %d is not valid.", start, end, len));
          }
          return new CharSlice(fb, off + start, l);
      }