Deprecate the concrete Alcotest.testable type

[craigfe]

The Alcotest API exposes the definition of Alcotest.testable as:

(** [TESTABLE] provides an abstract description for testable values. *)
module type TESTABLE = sig
  type t
  (** The type to test. *)

  val pp : t Fmt.t
  (** A way to pretty-print the value. *)

  val equal : t -> t -> bool
  (** Test for equality between two values. *)
end

type 'a testable = (module TESTABLE with type t = 'a)
(** The type for testable values. *)

val testable : 'a Fmt.t -> ('a -> 'a -> bool) -> 'a testable
(** [testable pp eq] is a new {!type-testable} with the pretty-printer [pp] and
    equality [eq]. *)

This is unfortunate, as it means testables can't keep any additional structure about the underlying type when it's available. For instance:

• there's no satisfying solution to neg combinator results in unhelpful failure logging #300 (i.e. without trying to pack extra information into equal / testable with global mutable state);
• it's not possible to add structural diffing in failing test output (as drafted by Subcomponent diffing #247), even when using only combinators from the Alcotest API itself.

We should deprecate the concreteness of the testable type and then make it abstract. Users will still be able to construct testables with Alcotest.testable, and to ease migration we can add a new function like Alcotest.testable_of_module that does something equivalent with first-class module syntax.