Introduce singleton types

Just for future, as this gives us a way to know immediately in the type, which represent normal, and which singleton classes
Also instantiate singleton class lazily (with singleton type)
This makes the type of class single, ie unique, immediately when it is used, ie methods or variables defined.
Fixes a design mistake, where all singletonn classes shared the same type, and thus unique methods per class were impossible
(Also some misc in commit)

test/risc/test_interpreter.rb

@@ -54,7 +54,7 @@ module Risc
     end
     def test_pc
       @interpreter.tick
-      assert_equal t = 37800 , @interpreter.pc
+      assert_equal t = 36104 , @interpreter.pc
       @interpreter.tick
       assert_equal t + 4 , @interpreter.pc
     end