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

tmp/test-node.js

@@ -16,6 +16,7 @@ ast.compute_char_frequency();
 UglifyJS.base54.sort();
 
 ast.figure_out_scope();
+ast.scope_warnings();
 ast.mangle_names();
 
 sys.error(UglifyJS.base54.get());