code: public int size() {
          if (pad_bits == 0) {
              return bin.length;
          }
          if (bin.length == 0) {
              throw new java.lang.IllegalStateException("Impossible length");
          }
          return bin.length - 1;
      }
docs: Get the size in whole bytes of the bitstr, rest bits in the last byte not
  counted.
  
  @return the number of bytes contained in the bintstr.