feat: add r/demo/atomicswap

[moul]

• implement
• unit test / txtar
• question: ugnot or grc20 or both? Maybe it’s time to encourage using wugnot.
  -> both, with a callback mechanism.
• question: p+r or just r?
  -> just r, and a single file. let's do it more!
• make the API gnokey compatible + add Render.

Depends on #2529 (cherry-picked)
Depends on #2549 (cherry-picked)
Depends on #2551
Closes #2549

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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[moul]

I think i'll try to be more grc20-centric for my demos. In involves writing some helpers to show how to actually come with gnot tokens, wrap then with wugnot. But also writing more and more grc20 examples: stupid ones, testing-purpose ones, etc.

[moul]

Cherry-picked commits from #3135 to verify if atomicswap functions with the current VM. (See #2743 (comment)).

[moul]

#3135 to be merged before this one.

[gh-gno-bot]

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