Add cstubs_applicative library

[mrmr1993]

This PR creates the cstubs_applicative library. The purpose of this is to allow us to bind or otherwise use cstubs bindings at their definition points, instead of needing a complex system with multiple layers. Intended uses are:

• ensuring that delete is always called upon garbage collection of foreign values
  • we currently have memory leaks that result from this
• allow OCaml types to be constructed directly at the bindings point from foreign record/struct types in a memory efficient way
• avoid duplication between the base interface and the OCaml-friendly interface.

For example:

let delete = foreign (prefix "delete") (typ @-> void)
let create = map2 (foreign (prefix "create") (A.typ @-> B.typ @-> typ)) delete ~f:(fun f delete ->
  let schedule_delete = Caml.Gc.finalise delete in
  fun ~a ~b ->
    let ret = f a b in
    schedule_delete ret ;
    ret

Checklist:

• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them:

[psteckler]

Our other .opam files should be this nice.

[mrmr1993]

Thanks. I plan to use this in the zexe submodule, so it needed to be opam pin-able for standalone use there, and it only seemed right to do it properly!

[imeckler]

Neat