Add NUT-XX: Token non-adjacent form (NAF)

[ok300]

This PR proposes a way to build a more compact token. For a table comparing it to the common token representation (powers of two), see https://gist.github.com/ok300/c94caa4b21807307bb21ab0dade38508 .

For easier reading: https://github.com/ok300/nuts/blob/ok300-token-naf/xx.md

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I'm not sure if this belongs as a new NUT, or as an addendum to NUT-02 "Keysets and fees", or as something else entirely. Arguments I considered:

• why it should be a NUT: because a mint that supports it will indicate it in its Mint Info response, which lets wallets know it supports more compact tokens without requiring wallets to fetch its keysets
• why it should be an addendum to NUT-02: to make mints and wallets aware of this more compact way of using keysets, since NUT-02 is focused on keysets

Please let me know if there is a better place for this, or if you find any issue with the reasoning in it.

[prusnak]

I don't get this proposal.

What is stopping me from swapping 4 sats for [4, -4, 4] and then swapping [4, 4] for [8] and discarding [-4]?

[ok300]

Didn't think of that, good catch. Yes, bad idea.