fix instruction positioning

use at as at and only compute difference when needed (to determine if
there is a jump in binary)
Easier to understand that way

lib/risc/text_writer.rb

@@ -72,7 +72,7 @@ module Risc
     def write_any( obj )
       write_any_log( obj ,  "Write")
       if @stream.length != Position.get(obj).at
-        puts "Write #{obj.class}:0x#{obj.object_id.to_s(16)} at 0x#{stream_position.to_s(16)} not #{Position.get(obj)}"
+        #puts "Write #{obj.class}:0x#{obj.object_id.to_s(16)} at 0x#{stream_position.to_s(16)} not #{Position.get(obj)}"
       end
       write_any_out(obj)
       write_any_log( obj ,  "Wrote")