[declare] Remove Proof_ending from the public API

This completes the refactoring [for now] of the core `Declare`
interface, and will allow much internal refactoring in the future.

In particular, we remove the low-level Proof_ending type, and instead
introduce higher-level constructors for the several declare users.

Future PRs will change the internal representation of proof handling
to better enforce some invariants that should hold for specific
proofs.

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