def: quasigroups (#453)

# Description

This PR defines defines quasigroups, and proves some very basic
properties about them.

## Checklist

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

- [x] I've read [the contributing
guidelines](https://github.com/plt-amy/1lab/blob/main/CONTRIBUTING.md).
- [x] The imports of new modules have been sorted with
`support/sort-imports.hs` (or `nix run --experimental-features
nix-command -f . sort-imports`).
- [x] All new code blocks have "agda" as their language.

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