a better fix for the 4.00.1 + existential type issue

The source of the problem was the Locate case that instantiates the
GADT parameter with a polymorphic variant:

```ocaml
type _ request =
| ...
| Locate
  :  string * position option
  -> [ `Found of string option * Lexing.position
     | `Not_in_env of string
     | `File_not_found of string
     | `Not_found
     ] request
```

Instantiation of rigid variables with polymorphic variant was
ill-supported in 4.00.1 and improved in 4.01, hence the
backward-incompatibility of the code before the first fix. The first
fix adds annotation everywhere, at a strong cost in concision and
readability; it is in fact sufficient to avoid "opening" the
polymorphic variant in the pattern-matching that does the
GADT-instantiation.

git diff HEAD~2 now gives:

```diff
      begin match request, response with
         | ...
         | Complete_prefix _, compl_list ->
           `List (List.map json_of_completion compl_list)
-        | Locate _, `Not_found ->
-          `String "Not found"
-        | Locate _, `Not_in_env str ->
-          `String (Printf.sprintf "Not in environment '%s'" str)
-        | Locate _, `File_not_found msg ->
-          `String msg
-        | Locate _, `Found (None,pos) ->
-          `Assoc ["pos",pos_to_json pos]
-        | Locate _, `Found (Some file,pos) ->
-          `Assoc ["file",`String file; "pos",pos_to_json pos]
+        | Locate _, resp ->
+          begin match resp with
+            | `Not_found -> `String "Not found"
+            | `Not_in_env str ->
+              `String (Printf.sprintf "Not in environment '%s'" str)
+            | `File_not_found msg ->
+              `String msg
+            | `Found (None,pos) ->
+              `Assoc ["pos",pos_to_json pos]
+            | `Found (Some file,pos) ->
+              `Assoc ["file",`String file; "pos",pos_to_json pos]
+          end
         | Drop, position -> pos_to_json position
         | Seek _, position -> pos_to_json position
         | Boundary _, Some {Location. loc_start; loc_end} ->
```