Description:
------------
When implementing an interface where a method is typehinted to return `self`, the return type in the implementing class must explicitly name the interface, rather than being allowed to use `self` as the typehint in the implementing class.

While the RFC (https://wiki.php.net/rfc/return_types) calls out hinting a child class as invalid, `self` on an implementing class seems like it should be valid, as `self` in the implementing class will, by definition, be an instance of the interface which is being implemented. Also, in practice, it makes writing out the code to implement the interface very tedious.

Test script:
---------------
// Does not work, unexpected:

<?php

interface Chainable {

    public function something(): self;

}

class MyChainable implements Chainable {

    public function something(): self {
        return $this;
    }

}

$x = new MyChainable();
print_r($x->something());

// Currently works (expected)
<?php

interface Chainable {

    public function something(): self;

}

class MyChainable implements Chainable {

    public function something(): Chainable{
        return $this;
    }

}

$x = new MyChainable();
print_r($x->something());

Expected result:
----------------
MyChainable Object
(
)


Actual result:
--------------
PHP Fatal error:  Declaration of MyChainable::something(): MyChainable must be compatible with Chainable::something(): Chainable in /Users/firehed/dev/7/ret.php on line 9