do not accept variables in constructor declarations.

Earlier we anticipated declaring projections in this way,
but never implemented it, and I still think this would
be a hack, not in line with the rest of the language.
Anyway, it's a three-line change, easily revertible.

src/parser/parser.mly

@@ -313,7 +313,7 @@ typecases:
 ;
 
 typecase:
-| uident params { (loc_i 1,$1,$2) }
+| uident primitive_types { (loc_i 1,$1,$2) }
 ;
 
 inddefn:

src/parser/ptree.mli

@@ -103,7 +103,7 @@ type param = ident option * pty
 type type_def = 
   | TDabstract
   | TDalias     of pty
-  | TDalgebraic of (loc * ident * param list) list
+  | TDalgebraic of (loc * ident * pty list) list
 
 type type_decl = {
   td_loc    : loc;

src/parser/typing.ml

@@ -718,7 +718,7 @@ let add_types dl th =
       | TDalgebraic cl ->
 	  let ty = ty_app ts (List.map ty_var ts.ts_args) in
 	  let constructor (loc, id, pl) =
-	    let param (_, t) = ty_of_dty (dty th' t) in
+	    let param t = ty_of_dty (dty th' t) in
 	    let tyl = List.map param pl in
 	    Hashtbl.replace csymbols id.id loc;
 	    create_fsymbol (id_user id.id loc) tyl ty