Simplex Category

[TOTBWF]

Description

This PR reworks the simplex category to make it more amenable to defining diagrams. It also proves a whole host of properties about it, which required the introduction of antiinjective and antisurjective functions. I was not able to find a better name for these, so suggestions are welcome!

Finally, this PR also introduces a bit of machinery for constructing concrete categories: a macro is used to introduce
top-level copattern matches so that we can get better goal display.

Checklist

Before submitting a merge request, please check the items below:

• I've read the contributing guidelines.
• The imports of new modules have been sorted with support/sort-imports.hs (or nix run --experimental-features nix-command -f . sort-imports).
• All new code blocks have "agda" as their language.

If your change affects many files without adding substantial content, and
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refactorings or reformattings), start the commit message and PR title with chore:.

[TOTBWF]

Just putting this up for a CI run, prose is still a bit rough

[Lavenza]

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