Proto: unabstract Contract.t

Context

As explained earlier, all the code already relies on the fact that contracts are already either implicit or originated. Then there's no reason to hide it.

It will allow specializing functions depending on whether they work on implicit accounts, smart contracts, or both.

As discussed offline with @klakplok, instead of going for the polymorphic variant or GADT solution, we'll keep the nested variants (public key hash / contract / destination) for now, wrap and unwrap as needed.

This is a prerequisite for #2803 (closed).

Previous related MRs: !5114 (merged), !5008 (merged)

Manually testing the MR

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Checklist

• Document the interface of any function added or modified (see the coding guidelines)
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• Provide automatic testing (see the testing guide).
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