Add polygon benchmarks

[adamreichold]

• [x] I agree to follow the project's code of conduct.

• [ ] I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

We used those to test different approaches at memoizing envelopes before settling on the combinators as the choice without hidden costs so we did not need them in the end. But I think they would still be useful to have in the long run.

[adamreichold]

bors r=urschrei

[bors[bot]]

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