Implement replacement and functional updates with lens-law checks

fieldglass.mli

@@ -3,3 +3,5 @@ type ('s, 'a) lens' = ('s, 's, 'a, 'a) lens
 
 val lens : view:('s -> 'a) -> set:('b -> 's -> 't) -> ('s, 't, 'a, 'b) lens
 val view : ('s, 't, 'a, 'b) lens -> 's -> 'a
+val set : ('s, 't, 'a, 'b) lens -> 'b -> 's -> 't
+val over : ('s, 't, 'a, 'b) lens -> f:('a -> 'b) -> 's -> 't