Add checksum to descriptor

[Sjors]

I was able to import the swap-in wallet descriptor into a Bitcoin Core watch-only wallet. But I had to manually add the checksum.

https://github.com/bitcoin/bips/blob/master/bip-0380.mediawiki#user-content-Checksum

[t-bast]

Right, we initially didn't have the checksum code written yet, and didn't update to include it, but that should be easy to do.