code: @Override
      public FastStr trim() {
          char[] value = this.buf;
          int len = end;
          int st = begin;
          char[] val = value;    /* avoid getfield opcode */
  
          while ((st < len) && (val[st] <= ' ')) {
              st++;
          }
          while ((st < len) && (val[len - 1] <= ' ')) {
              len--;
          }
          return ((st > begin) || (len < size())) ? new FastStr(value, st, len) : this;
      }
docs: Wrapper of {@link String#trim()} and return FastStr type instance
  
  @return a FastStr instance without leading and tail space characters
  from this FastStr instance