Port z_listunspent

[leto]

Ported from https://github.com/zcash/zcash/pull/2913 with a few tiny fixes.

One thing we may want to change is that it shows amount=0 notes, which will become very common. It would be nice to filter those out.

Tested on Hush mainnet, works for me on Linux.

  {
    "txid": "deadbeef81335a2ecf218c338ae11941a6bdff43b4070a69b1df87e78968e5f74",
    "js_index": 0,
    "output_index": 1,
    "confirmations": 8733,
    "spendable": true,
    "address": "zcTooManySecretsHgmMLToHEcStDRcwW8S3TDeWbRiEiqYjHax62c",
    "amount": 0.00420500,
    "memo": "0420198702000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
  }

[FireMartZ]

Looks good!