Generalize pubkey checksum display and check and introduce `checksum` command
- [x] #333: Always display a pubkey with its associated checksum - [x] Check checksum validity when a pubkey is passed - [x] Add new `checksum` command: you pass the pubkey and then it returns: `<pubkey>:>checksum>` - [ ] #302: Checksum mandate when passing a pubkey
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