Define polymorphic lenses and compile the runtime library

fieldglass.ml

@@ -0,0 +1,5 @@
+type ('s, 't, 'a, 'b) lens = ('s -> 'a) * ('b -> 's -> 't)
+type ('s, 'a) lens' = ('s, 's, 'a, 'a) lens
+
+let lens ~view ~set = view, set
+let view (get, _) = get