Provide the identity lens and verify composition identities

fieldglass.ml

@@ -12,3 +12,4 @@ let compose (outer_get, outer_put) (inner_get, inner_put) =
 
 let _1 = (fun (left, _) -> left), (fun left (_, right) -> left, right)
 let _2 = (fun (_, right) -> right), (fun right (left, _) -> left, right)
+let _id = (fun value -> value), (fun value _ -> value)