Module `Higher_kinded`

A minimal higher-kinded library for the purpose of tests and tutorials.

This is a reduced variation on https://github.com/janestreet/higher_kinded that we use to explore ways the Provider library may be used with interfaces containing parametrized types.

We use this small kernel rather than an upstream library because we needed to add some injectivity annotations to the types, and the higher-kinded libraries available do not have them at this time. Proposing this change upstream would require more thoughts, and so far we didn't have actual usage for this (outside of tests and tutorials), so we went with this small kernel instead.

type !'a t

type !'a hk := 'a t

module type S = sig ... end

module Make (X : sig ... end) : S with type 'a t := 'a X.t