Add meta assembly commands for dealing with variables

asm/fib.scm

@@ -1,6 +1,5 @@
   (variable result 0)
-  (push result)
-  (@)
+  (ref result)
   (push fib)
   (call)
   (push cleanup)
@@ -31,6 +30,5 @@ recur
   (nip)
   (return)
 cleanup
-  (push result)
-  (!)
+  (set! result)
   (bye)