fixed some mess with symbols/scope

- all symbols now have a `thedef` property which is a SymbolDef object,
  instead of the `uniq` that we had before (pointing to the first occurrence
  of the name as declaration).

- for undeclared symbols we still create a SymbolDef object in the toplevel
  scope but mark it "undeclared"

- we can now call figure_out_scope after squeezing, which is useful in order
  not to mangle names that were dropped by the squeezer

lib/output.js

@@ -884,16 +884,9 @@ function OutputStream(options) {
         self.value._do_print(output, true);
     });
     DEFPRINT(AST_Symbol, function(self, output){
-        output.print_name(self.name);
-    });
-    DEFPRINT(AST_SymbolDeclaration, function(self, output){
         var def = self.definition();
         output.print_name(def.mangled_name || def.name);
     });
-    DEFPRINT(AST_SymbolRef, function(self, output){
-        var def = self.symbol;
-        output.print_name(def ? def.mangled_name || def.name : self.name);
-    });
     DEFPRINT(AST_This, function(self, output){
         output.print("this");
     });