[obligations] Pre-functionalize Program state

In our quest to unify all the declaration paths, an important step
is to account for the state pertaining to `Program` declarations.

Whereas regular proofs keep are kept in a stack-like structure;
obligations for constants defined by `Program` are stored in a global
map which is manipulated by almost regular open/close proof primitives.

This PR is in preparation for the switch to a purely functional state
in #11836 ; the full switch requires deeper changes so it is helpful
to have this PR preparing most of the structure.

Most of the PR is routine; only remarkable change is that the hook for
admitted obligations is now called explicitly in `finish_admitted` as
it had to learn about the different types of proof_endings. Before,
obligations set it in `start_lemma` but only used in the `Admitted`
path.

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