Support unserializing broker data into type 'any'

The receiver side will wrap the data as a Broker::Data value, which
can then be type-checked/cast via 'is' or 'as' operators to a specific
Bro type.  For example:

Sender:

    Broker::publish("topic", my_event, "hello")

Receiver:

    event my_event(arg: any)
        {
        if ( arg is string )
            print arg as string;
        }

src/broker/Data.cc

@@ -772,6 +772,9 @@ struct type_checker {
 
 Val* bro_broker::data_to_val(broker::data d, BroType* type)
 	{
+	if ( type->Tag() == TYPE_ANY )
+		return bro_broker::make_data_val(move(d));
+
 	return broker::visit(val_converter{type}, std::move(d));
 	}