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/compress.js

@@ -991,7 +991,7 @@ merge(Compressor.prototype, {
     /* -----[ optimizers ]----- */
 
     OPT(AST_Directive, function(self, compressor){
-        if (self.scope.has_directive(self.value) !== self.scope) {
+        if (compressor.has_directive(self.value) === "up") {
             return make_node(AST_EmptyStatement, self);
         }
         return self;