Fix Rational.norm

It assumes the Rationals are always reduced, however, that's not the
case for optimization reasons.

Fix by reducing before returning self.

Bug find: https://irclog.perlgeek.de/perl6/2017-02-13#i_14092899

src/core/Rational.pm

@@ -201,7 +201,7 @@ my role Rational[::NuT, ::DeT] does Real {
         self.new($!numerator - $!denominator, $!denominator);
     }
 
-    method norm() { self }
+    method norm() { self.REDUCE-ME; self }
 
     method narrow(::?CLASS:D:) {
         self.REDUCE-ME;