Process attributes after Qed/Defined

[gmalecha-at-skylabs]

This PR extends the handling of attributes so that handlers are executed when the term is completed, e.g. at the Qed or Defined.

It uses this feature to implement a plugin that provides an attribute #[hint(db=...,cost=...,visibility=...)]. This should be either incorporated into core Rocq or introduced as a separate plugin.

For now, I'm just looking for feedback, the commits should be independently reviewable. The last commit contains the hint attribute implementation and could probably use more testing.

• Added / updated test-suite.

• Added changelog.
• Added / updated documentation.
  • Documented any new / changed user messages.
  • Updated documented syntax by running make doc_gram_rsts.

• Opened overlay pull requests.

[gmalecha-at-skylabs]

@SkySkimmer What should the next steps here be?

1. I think it would be really useful to get the core part of the change in so that attributes on Lemma and whatnot work well. The only concern is that you could run into an error on Qed after you do a big proof.
2. What do you think about the code that actually implements the attribute? Should it be placed in a plugin that is distributed separately?

I'm happy to put more cycles into the development.