[part3] Abstract Run_state behind Backend

[mimoo]

This builds on top of https://github.com/o1-labs/snarky/pull/793 to remove the run_state and the constraint_system from snarky, and offer it through the backend.

It does that in a very naive way, and:

• still doesn't address the TODO of https://github.com/o1-labs/snarky/pull/793
• don't extract the global state part of the state to the ocaml side (yet)

the next steps:

• [ ] make this work on the Mina side (in progress: https://github.com/MinaProtocol/mina/pull/12738)
• [ ] address the TODO of https://github.com/o1-labs/snarky/pull/793
• [ ] make sure we pass the tests
• [ ] then we can talk about recreating some run_state on this side
• [ ] and moving more logic (constraints stuff) to do the rust side

[github-actions[bot]]

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