avoid slow IntInf.pow in Poly/ML 5.7.1 testing version, e.g. relevant for AFP/Lorenz_C0;

--- a/src/Pure/General/integer.ML	Fri Nov 03 19:16:41 2017 +0100
+++ b/src/Pure/General/integer.ML	Fri Nov 03 19:20:47 2017 +0100
@@ -65,3 +65,10 @@
   | lcms (x :: xs) = abs (Library.foldl PolyML.IntInf.lcm (x, xs));
 
 end;
+
+(* FIXME workaround for Poly/ML 5.7.1 testing *)
+structure IntInf =
+struct
+  open IntInf;
+  fun pow (i, n) = Integer.pow n i;
+end