Rework has_directive

It's now available during tree walking, i.e. walker.has_directive("use
asm"), rather than as part of the scope.  It's thus no longer necessary
to call `figure_out_scope` before codegen.  Added special bits in the
code generator to overcome the fact that it doesn't inherit from
TreeWalker.

Fix #861

lib/transform.js

@@ -70,7 +70,7 @@ TreeTransformer.prototype = new TreeWalker;
                     if (y !== undefined) x = y;
                 }
             }
-            tw.pop();
+            tw.pop(this);
             return x;
         });
     };