move all position setting into position

Position and subclasses handle the logic, external to
the classes, so it can be swapped out later
(at runtime positions can’t change)

test/risc/test_padding.rb

@@ -42,7 +42,7 @@ module Risc
     end
     def test_pos_arm
       mov = Arm::ArmMachine.mov  :r1,  128
-      mov.set_position(0,0)
+      Risc::Position.set(0,0)
     end
 
   end