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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.95%. Comparing base (0dc4b6d) to head (b05e615).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2510      +/-   ##
==========================================
- Coverage   54.96%   54.95%   -0.01%     
==========================================
  Files         595      595              
  Lines       79568    79568              
==========================================
- Hits        43731    43726       -5     
- Misses      32529    32533       +4     
- Partials     3308     3309       +1     

Flag	Coverage Δ
contribs/gnodev	25.65% <ø> (-0.35%)	⬇️
contribs/gnofaucet	15.31% <ø> (ø)
contribs/gnokeykc	0.00% <ø> (ø)
contribs/gnomd	0.00% <ø> (ø)
gno.land	63.14% <ø> (ø)
gnovm	60.31% <ø> (ø)

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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.