This commit implements property-specific accessors.

This commit implements property-specific accessors.

When overload() is called, it not only scans for general handlers, but
also for any methods beginning with __get_ and __set_ and saves a list of
them. Then when a property is accessed, e.g. $obj->x, the extension will first
check whether __get_x() is available, and will call it if it is, otherwise
it will invoke the general __get() handler.