CI: Refactor build by extracting contracts

Depends on !115 (merged) .

Changes ./configure in root so that a subset of the project can be built.

Changes the opam so that contracts are not verified when installing. Currently, I do not extract mi-cho-cott in a separate opam file but that is possible.

Adds a separate for job for verifying the contracts. This job only runs with Coq 8.11.

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