phase 2, add/sub instructions work with +- operator

test/arm/test_small_program.rb

@@ -13,15 +13,13 @@ class TestSmallProg < MiniTest::Test
   end
 
   def test_loop
-    start = Vm::Block.new("start")
+    s = Vm::Block.new("start").scope binding
     m = @program.main.scope binding
     r0 = Vm::Integer.new(0)
     m.r0 = 5                #1
-    m << start
-    start.instance_eval do
-      sub  :r0, :r0, 1 , :update_status => 1      #2
-      bne  start          #3
-    end
+    m << s
+    s.r0 = r0 - 1      #2
+    s.bne  s          #3
     @should = [0,176,160,227,5,0,160,227,1,0,80,226,253,255,255,26,1,112,160,227,0,0,0,239]
     write "loop"
   end