[part4] Changes to snarky to make snarky-rs integration work

[mimoo]

Builds on top of https://github.com/o1-labs/snarky/pull/798

This contains a number of misc changes that I need to make in order to make the Mina-side work (https://github.com/MinaProtocol/mina/pull/12738). It contains:

• remove length, to_constant_and_terms, var_indices, linear_combination, sum from Cvar (unused)
• makes sure that the reference fields of run_state are not impacted (explained in this PR: https://github.com/o1-labs/snarky/pull/800)
• create a wrapper state around the rust state (in state.ml)
• delete unused fake_state function
• make OCaml code use what's passed from backend (state, cvar)
• clean up the State.make function
• introduce the seal function that exists in pickles (so we can delete it there)

the wrapper state has moved anything that's not relevant to Rust to its own fields:

  type t =
    { state : Run_state.t
    ; stack : string list
    ; handler : Request.Handler.t
    ; is_running : bool
    ; log_constraint :
        (   ?at_label_boundary:[ `Start | `End ] * string
         -> (Cvar.t, Field.t) Constraint.t option
         -> unit )
        option
    }

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