Use xor to avoid allocations in `AR::Core#hash`

This is not as good a solution as actually hashing both values, but Ruby
doesn't expose that capability other than allocating the array. Unless we were
to do something silly like have a thread local array that is re-used, I don't
see any other way to do this without allocation. This solution may not be
perfect, but it should reasonably avoid collisions to the extent that we need.

activerecord/lib/active_record/core.rb

@@ -452,7 +452,7 @@ def ==(comparison_object)
     #   [ Person.find(1), Person.find(2), Person.find(3) ] & [ Person.find(1), Person.find(4) ] # => [ Person.find(1) ]
     def hash
       if id
-        [self.class, id].hash
+        self.class.hash ^ self.id.hash
       else
         super
       end