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/Attr.cc

@@ -260,6 +260,11 @@ void Attributes::CheckAttr(Attr* a)
 				// Ok.
 				break;
 
+			if ( type->Tag() == TYPE_TABLE &&
+			     type->AsTableType()->IsUnspecifiedTable() )
+				// Ok.
+				break;
+
 			a->AttrExpr()->Error("&default value has inconsistent type", type);
 			}
 
@@ -290,6 +295,11 @@ void Attributes::CheckAttr(Attr* a)
 					// Ok.
 					break;
 
+				Expr* e = a->AttrExpr();
+				if ( check_and_promote_expr(e, ytype) )
+					// Ok.
+					break;
+
 				Error("&default value has inconsistent type 2");
 				}