Fix issue #680 with encoding int-keyed maps.

[herczegzsolt]

Hi!

I've created this pull request to address #680. I can confirm that it works for my use-case, but I'm not sure if it has any side-effects or what would be the best way to write tests for it.

If this need more work, i'll come back to it at a later date. Any guidance on the expected tests would be greatly appreciated in the meantime.

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Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.85%. Comparing base (50b37c1) to head (0559ad4).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #681      +/-   ##
==========================================
- Coverage   77.87%   77.85%   -0.03%     
==========================================
  Files          22       22              
  Lines        7960     7965       +5     
==========================================
+ Hits         6199     6201       +2     
- Misses       1348     1350       +2     
- Partials      413      414       +1     

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[herczegzsolt]

I believe that the encoding of struct-keyed maps is also incorrect. For example:
https://go.dev/play/p/EyVVqlViyUN

I've created a fix for simple types that already implement ast.MapKeyNode, but i'm not sure what to do about others.