Add callback-oriented versions of constraint_system and generate_witness

[mrmr1993]

This PR refactors the internals of snarky to expose a callback-oriented version of the functions constraint_system and generate_witness. As the core entrypoints used by pickles, this allows us to use these callbacks to handle async witness generation.

This PR is structured as a series of refactoring commits, plus a final commit that implements the new functions. Each refactoring is deliberately structured so that there is no behavioural change, except for a few small performance optimisations as a result of removing duplicated/redundant computations.

[mitschabaude]

to let callers recover from errors, we could add a third callback, which just resets the state to what it was:

type ('input_var, 'return_var, 'result) manual_callbacks =
      { run_circuit : 'a. ('input_var -> unit -> 'a) -> 'a
      ; finish_computation : 'return_var -> 'result
      ; reset_state : unit -> unit
      }

and you'd use it like this pseudo-code

let builder = constraint_system_manual();
try {
  let result = await build.run_circuit(input);
  builder.finish_computation(result);
} catch (err) {
  builder.reset_state();
}

since its optional to use use reset_state and handle errors, this would be an easy change.

still approving though because it seems that the present version is good enough (passes all tests across the stack, even those that intentionally create errors in circuits - probably because snarky's current state, however broken, never affects subsequent circuit runs, because states are just nested)

[mitschabaude]

see my review on a way to improve this