In PHP 8, but not in PHP 7.4, every class with a __toString() function
implicitly implements the Stringable interface. Therefore, the
behavior of checks like "instanceof Stringable" differs between these
PHP versions when such classes are involved. Make every such class
implement the interface so that the behavior will be consistent.
The PHP 7.4 fallback for the Stringable interface is provided by
symfony/polyfill-php80.
Change-Id: I3f0330c2555c7d3bf99b654ed3c0b0303e257ea1
ccd423225f