function fromUInt64 (buf) {
  const high = buf.readUInt32BE(0) | 0 // force
  const low = buf.readUInt32BE(4) | 0
  const lowUnsigned = (low >= 0) ? low : TWO_PWR_32 + low

  return (high * TWO_PWR_32) + lowUnsigned
}
Return the closest floating-point representation to the buffer value. Precision will be
lost for big numbers.