feat: add toId(hdkey)

[coolaj86]

As per dashpay/dips#134 (comment).

DashHd.toIdBytes = async function (hdkey) {
  let xpubBytes = await DashHd.toXPubBytes(hdkey);

  let hashBytes = await crypto.subtle.digest("SHA-256", xpubBytes)
  let idBytes = hashBytes.slice(0, 8);
  return idBytes;
}

let hdkey = await DashHd.fromXKey(xkey);
let idBytes = DashHd.toIdBytes(hdkey);

let id = bytesToBase64Url(idBytes);
let idBase32Crock = base32crockford.encode(idBytes);
let idHex = DashKeys.utils.bytesToHex(idBytes);
let idInt64 = BigInt(`0x${idHex}`);