Pin Why3's version to a specific hash & refactor CI

[Armael]

This PR does a number of related things to improve the setup workflow for users:

• introduce a creusot-deps.opam file, to allow creating the required opam switch with one command
• in the opam file, pin why3 to a specific "known good" commit, so that users don't pull an unknown version on why3 at setup
• refactor the CI workflows to:
  • use the opam file with the "known good" commit on master/PRs
  • have an extra nightly workflow to test creusot against why3 master

The idea is then that upgrading the "known good" why3 commit is a manual operation (just editing the opam file), that I documented in HACKING.md.

[jhjourdan]

Very nice, I'd like to merge ASAP, if @xldenis agrees.

[xldenis]

Yep, this looks good to me.