Provide the identity lens and verify composition identities

fieldglass.mli

@@ -8,3 +8,4 @@ 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
+val _id : ('a, 'b, 'a, 'b) lens