Add polymorphic tuple projections and replacements

fieldglass.mli

@@ -6,3 +6,5 @@ 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
 val compose : ('s, 't, 'a, 'b) lens -> ('a, 'b, 'c, 'd) lens -> ('s, 't, 'c, 'd) lens
+val _1 : ('a * 'c, 'b * 'c, 'a, 'b) lens
+val _2 : ('c * 'a, 'c * 'b, 'a, 'b) lens