start with #14 by implementing factory

page was maybe a too low level name
pages may be the unit of the syscall, but after that objects desolve (maybe later to be added on from different pages)
Factory has the job of handing out a new instance of a type
it keeps a freelist for that and a reserve

test/risc/interpreter/calling/test_minus.rb

@@ -45,7 +45,7 @@ module Risc
       ret = main_ticks(64)
       assert_equal FunctionReturn ,  ret.class
       assert_equal :r1 ,  ret.register.symbol
-      assert_equal 26472 ,  @interpreter.get_register(ret.register)
+      assert_equal 27080 ,  @interpreter.get_register(ret.register)
     end
   end
 end