Added support for PHP 8.0 promoted parameters

[michaelpetri]

Q	A
Documentation	no
Bugfix	no
BC Break	no
New Feature	yes
RFC	yes
QA	no

Description

Adds support for promoted parameters

Solves (partially) #104

[Ocramius]

Discussed this review in a 1-on-1 session: PHP 8.1 is currently a bit out of reach.

<?php

interface Bar {}
class Baz implements Bar {}
class Foo
{
    public function __construct(
        public Bar $bar = new Baz()
    ) {}
    
    public function doThing(Bar $bar = new Baz()) {
        return $bar;
    }
}

var_dump(new Foo());

var_dump((new Foo)->doThing());

https://3v4l.org/SoOPU

In practice, we should stop this patch at only supporting initializers (and visibility thereof), while we will skip new_in_initializers for now ( https://wiki.php.net/rfc/new_in_initializers ) because reflection for it does not give us enough information about the default value yet.

We will likely need AST-based reflector for that, and we're far from that: at that point, this library will either need phasing out, or a full rewrite.

[Ocramius]

LGTM, excellent work, @michaelpetri!