Add polymorphic tuple projections and replacements

fieldglass.ml

@@ -9,3 +9,6 @@ let over (get, put) ~f source = put (f (get source)) source
 let compose (outer_get, outer_put) (inner_get, inner_put) =
   (fun source -> inner_get (outer_get source)),
   (fun value source -> outer_put (inner_put value (outer_get source)) source)
+
+let _1 = (fun (left, _) -> left), (fun left (_, right) -> left, right)
+let _2 = (fun (_, right) -> right), (fun right (left, _) -> left, right)