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

test/risc/position/test_instruction_position.rb

@@ -7,6 +7,7 @@ module Risc
       def setup
         Risc.machine.boot
         @binary = Parfait::BinaryCode.new(1)
+        Position.set(@binary , 0)
         @label = Label.new("hi","ho")
       end
       def test_set_instr