[RFC] Differentiate op from assign-op in operator overloading

This is a separate proposal from the userspace operator overloading I put up for Patricio yesterday and aims to fix what I see as a bug in our operator overloading implementation (though some may disagree). It specifically only seeks to differentiate const operations which produce a new value from mutations which alter an existing overloaded object. https://wiki.php.net/rfc/assignment-overloading