groundwork for interpreter platform

especially positioning makes the previous interpreter approach to
fragile (too far off the real thing)
Interpreter should be it’s own platform (still use the risc
instructions),
get a translator and use positions

test/risc/test_text_writer.rb

@@ -18,6 +18,7 @@ module Risc
 
     def setup
       @machine = Risc.machine.boot
+      @machine.translate(:arm)
       @machine.position_all
       @text_writer = TextWriter.new(@machine)
     end