refactor(engine): complete cleanup of ast_utils

AST utils used to be one monolithic file without good organization and
with no documentation at all.

This commit drops `ast_utils.ml` in favor of `ast_utils/*` modules,
and reoganize things.

I intend on documenting everything in `ast_utils/*`.

engine/lib/analyses/function_dependency.ml

@@ -14,7 +14,7 @@ module%inlined_contents Make (F : Features.T) = struct
  Ast_utils.MakeWithNamePolicy (F) (Concrete_ident.DefaultNamePolicy)
 
  let analyse (items : A.item list) : analysis_data =
- let temp_list = List.concat_map ~f:U.functions_of_item items in
+ let temp_list = List.concat_map ~f:U.Destruct.Item.functions items in
  List.fold_left
  ~init:(Map.empty (module String))
  ~f:(fun y (name, body) ->

engine/lib/analyses/mutable_variables.ml

@@ -164,7 +164,7 @@ module%inlined_contents Make (F : Features.T) = struct
  ~f:(fun (y, count) (name, body) ->
  let items, count = analyse_function_body body count in
  (y @ [ (name, items) ], count))
- (List.concat_map ~f:U.functions_of_item items)
+ (List.concat_map ~f:U.Destruct.Item.functions items)
  in
  let mut_map (* Concrete_ident *) :
  (Local_ident.t list * (U.TypedLocalIdent.t * id_order) list)

engine/lib/ast.ml

@@ -629,7 +629,7 @@ functor
  (s : string) : item =
  { v = HaxError s; span; ident; attrs = [] }
 
- module F = F
+ (* module F = F *)
  end
 
 module type T = sig