Fix various bugs with table/set attributes.

- Identifiers that are initialized with set()/table() constructor
  expressions now inherit attributes from the expression.  Before,
  statements like

     const i: set[string] = set() &redef;

  associated the attribute with the set() constructor, but not the
  "i" identifier, preventing redefinition.  Addresses #866.

- Allow &default attribute to apply to tables initialized as empty
  (via either "{ }" or "table()") or if the expression supplied to it
  can evaluate to a type that's promotable to the same yield type as
  the table.

src/Expr.h

@@ -747,6 +747,8 @@ public:
 	TableConstructorExpr(ListExpr* constructor_list, attr_list* attrs);
 	~TableConstructorExpr()	{ Unref(attrs); }
 
+	Attributes* Attrs() { return attrs; }
+
 	Val* Eval(Frame* f) const;
 
 protected:
@@ -767,6 +769,8 @@ public:
 	SetConstructorExpr(ListExpr* constructor_list, attr_list* attrs);
 	~SetConstructorExpr()	{ Unref(attrs); }
 
+	Attributes* Attrs() { return attrs; }
+
 	Val* Eval(Frame* f) const;
 
 protected: