Replace K__ with tuple, more informative

lib/Monomorphization.ml

@@ -72,12 +72,13 @@ module NameGen = struct
     in
     Common.Comment (KPrint.bsprintf "%a" PrintCommon.pdoc comment)
 
-  let gen_lid ((m, n) as lid) ts extra =
+  let gen_lid lid ts extra =
     if !short_names then
       if lid = tuple_lid && List.for_all ((=) (List.hd ts)) ts then
         let n = KPrint.bsprintf "%a_x%d" ptyp (List.hd ts) (List.length ts) in
         ([], n), [ Common.AutoGenerated ]
       else
+        let m, n = if lid = tuple_lid then [], "tuple" else lid in
         let hash = Hashtbl.hash (ts, extra) in
         let n = Printf.sprintf "%s_%02x" n (hash land 0xFF) in
         let n = Idents.mk_fresh n (fun n -> Hashtbl.mem seen (m, n)) in