Add polymorphic tuple projections and replacements

runtime.ml

@@ -20,3 +20,8 @@ let () =
   let focus = compose first second in
   assert (view focus ((true, "grey"), false) = "grey");
   assert (set focus 9 ((true, "grey"), false) = ((true, 9), false))
+
+let () =
+  assert (set _1 "grey" (1, true) = ("grey", true));
+  assert (over _2 ~f:string_of_int (false, 9) = (false, "9"));
+  assert (view (compose _1 _2) ((true, "grey"), ()) = "grey")