Just as a heads up, I rebased and force pushed.

Regarding the automatic implementation of Stringable, I initially thought we could simply add this at the very end of inheritance ... but then I realized that this automatic implementation could be relevant for variance checks:

class Foo {
    public function test(): Stringable {}
}
class Bar extends Foo {
    public function test(): Bar {}
    public function __toString() {}
}

Which would be legal if Bar automatically implements Stringable. To allow that, we need to add it as a proper interface during initial compilation.