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/test_linker.rb

@@ -25,7 +25,7 @@ module Risc
       assert_equal 0 ,  Position.get(@linker.cpu_init).at
     end
     def test_cpu_at
-      assert_equal "0x74cc" ,  Position.get(@linker.cpu_init.first).to_s
+      assert_equal "0x76ec" ,  Position.get(@linker.cpu_init.first).to_s
     end
     def test_cpu_label
       assert_equal Position ,  Position.get(@linker.cpu_init.first).class