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/ast.js

@@ -549,11 +549,11 @@ var AST_ObjectGetter = DEFNODE("ObjectGetter", null, {
     $documentation: "An object getter property",
 }, AST_ObjectProperty);
 
-var AST_Symbol = DEFNODE("Symbol", "scope name global undeclared", {
+var AST_Symbol = DEFNODE("Symbol", "scope name thedef", {
     $documentation: "Base class for all symbols",
 });
 
-var AST_SymbolDeclaration = DEFNODE("SymbolDeclaration", "references", {
+var AST_SymbolDeclaration = DEFNODE("SymbolDeclaration", null, {
     $documentation: "A declaration symbol (symbol in var/const, function name or argument, symbol in catch)",
 }, AST_Symbol);
 
@@ -581,7 +581,7 @@ var AST_Label = DEFNODE("Label", null, {
     $documentation: "Symbol naming a label (declaration)",
 }, AST_SymbolDeclaration);
 
-var AST_SymbolRef = DEFNODE("SymbolRef", "symbol", {
+var AST_SymbolRef = DEFNODE("SymbolRef", null, {
     $documentation: "Reference to some symbol (not definition/declaration)",
 }, AST_Symbol);