4.12 injectivity annotations aren't (yet) supported

[dra27]

ocaml/ocaml#9500 adds support for injectivity annotations on abstract types - at present odoc can't build with trunk OCaml, therefore, since Asttypes.variance changed.

[dra27]

A very small hack:

--- a/src/loader/cmti.ml
+++ b/src/loader/cmti.ml
@@ -178,9 +178,9 @@ let read_type_parameter (ctyp, var) =
   in
   let var =
     match var with
-    | Covariant -> Some Pos
-    | Contravariant -> Some Neg
-    | Invariant -> None
+    | Covariant, _ -> Some Pos
+    | Contravariant, _ -> Some Neg
+    | NoVariance, _ -> None

and a revert of ocaml/ocaml#9781 allows odoc master to compile trunk OCaml's documentation.