n-byte type and length (3-bit type, (n-1)*7+4-bit length) CTTTSSSS C is continue bit, TTT is type, S+ is length
function $header(byte) {
    if (start === 0) start = position;
    type = byte >> 4 & 0x07;
    length = byte & 0x0f;
    if (byte & 0x80) {
      offset = 4;
      return $header2;
    }
    return afterHeader();
  }