Implement replacement and functional updates with lens-law checks

fieldglass.ml

@@ -3,3 +3,5 @@ type ('s, 'a) lens' = ('s, 's, 'a, 'a) lens
 
 let lens ~view ~set = view, set
 let view (get, _) = get
+let set (_, put) = put
+let over (get, put) ~f source = put (f (get source)) source