improve SymbolDef info in --output ast (#2778)

* SymbolDef info (a.k.a. `thedef`) is now represented as a string containing `"ID name [mangled_name]"`. 
* Enhance display of `globals`, `variables`, `functions` and `enclosed`.
* `SymbolDef.next_id` starts at `1` and the `id` is adjusted for `-o ast` display.

lib/scope.js

@@ -57,7 +57,7 @@ function SymbolDef(scope, orig, init) {
     this.id = SymbolDef.next_id++;
 };
 
-SymbolDef.next_id = 1e6;
+SymbolDef.next_id = 1;
 
 SymbolDef.prototype = {
     unmangleable: function(options) {