Generalize pubkey checksum display and check and introduce `checksum` command

#333 (closed): Always display a pubkey with its associated checksum
Check checksum validity when a pubkey is passed
Add new checksum command: you pass the pubkey and then it returns: <pubkey>:>checksum>
#302 (closed): Checksum mandate when passing a pubkey

Edited Oct 13, 2020 by Moul

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