Signet update

Hello,

Some progress (courtesy of ajtowns!) has been made on the Signet proposal (BIP-325):

* Block signatures are now verified as transactions

Among other things, this means Signet is (probably) compatible with PSBT (thanks sipa!), and no longer requires the custom "simple signature verifier/creator" stuff.

This unfortunately means Signet will hard fork, but I don't think anyone is using it seriously yet, so it shouldn't be an issue.

The BIP pull request is here: https://github.com/bitcoin/bips/pull/947

Feedback welcome.