Put projectors into web then quickly pull it out

[disconcision]

This represents a trial attempt to move all of core (after semantics was extracted) into web, and then pull projectors out (wanted to get an idea if a functorial approach might work well here). I created a module type representing projectors current syntax dependency, to use as a kind of stand-in for a future Editor.t dependency:

module type Syntax = {
  type segment;
  let unparenthesize: segment => segment;
  let to_string: segment => string;
  let sort_of: segment => Sort.t;
};

(these are all things projectors currently use, somewhat incidentally)

Then i turned ProjectorBase.Projector into a functor taking that as an argument:

module type Projector =
  (Syntax: Syntax) =>
   {
    /* The internal model type of the projector which will
     * be serialized and persisted. Use `unit` if you don't
     * need other state beyond the underlying syntax */
    [@deriving (show({with_path: false}), sexp, yojson)]
    type model;
    ...

and the implementations get functorized as well:

module Make: Projector =
  (
    Syntax: {
      type segment;
      let unparenthesize: segment => segment;
      let sort_of: segment => Semantics.Sort.t;
    },
  ) => {
    [@deriving (show({with_path: false}), sexp, yojson)]
    type model = t;
    ...

BUT with the above i can't figure out how to reinstate the GADT type constraints we previouslu introduced here e.g. with model = ProjectorCore.Kind.fold_model

I got as far as trying to functorize the to_module functionality:

module Init = (Syntax: Syntax) => {
  let to_module =
      (type a, kind: ProjectorCore.Kind.gadt(a))
      : (module ProjectorInstance with type model = a) => {
    switch (kind) {
    | Fold => (module FoldProj.Make(Syntax))
    | Info => (module TypeProj.Make(Syntax))
    ...

but now I start running into, I think, that effect of removing that type constraint:

This expression has type
  (module ProjectorInstance with type model = FoldProj.Make(Syntax).model)
but an expression was expected of type
  (module ProjectorInstance with type model = a)
Type FoldProj.Make(Syntax).model is not compatible with type a = FoldProj.t