Add nominal features

[MarkusKL]

Ready for review.

Closes #54
Closes #55
(replaced by case-study in examples).

[spitters]

Remember to look at whether we can use nominal Rocq and finmap permutations.
#66

[4ever2]

You can ignore the opam workflow failure it will be fixed in #81

[spitters]

@4ever2 @cmester0 let's get this merged. Have you looked at it already?
I'll try to have a look tomorrow.

[spitters]

I think this should be merged.
@4ever2 @MarkusKL do you understand the skipped opam build in CI? Ideally, we'd like to resolve it.

[4ever2]

do you understand the skipped opam build in CI? Ideally, we'd like to resolve it.

That is normal, it runs after merging the PR.

[MarkusKL]

Thanks for the fix. I am ready.