code: function encode(buffer, extra) {
    if (!extra) extra = 0
    var blen = buffer.length
    var lenbytes = vencode(blen)
    var mb = new Buffer(extra + blen + lenbytes.length)
    for (var i = 0; i < lenbytes.length; i++) {
      mb.writeUInt8(lenbytes[i], extra + i)
    }
    buffer.copy(mb, lenbytes.length + extra, 0, blen)
    return mb
  }
docs: Encode a buffer with a varint prefix containing the buffer's length.
  @param  {Buffer} buffer The buffer to encode
  @param  {int} extra How many extra empty leading bytes to put in the buffer
  @return {Buffer}        An encoded buffer longer than before.