Adds Function Attributes support

[nunomaduro]

This pull request adds function attributes support to serialisable closures:

#[Attribute(Attribute::TARGET_FUNCTION)]
class MyAttribute
{
    // ..
}

$closure = #[MyAttribute('My attribute argument')] function () {
    return true;
};

$closure = unserialize(serialize($closure))->getClosure();

// Before this pull request:
dd(new ReflectionFunction($closure))->getAttributes());
// []

// After this pull request:
dd(new ReflectionFunction($closure))->getAttributes());
// [['name' => 'MyAttribute', 'arguments' => ['My attribute argument']]]

[morloderex]

@nunomaduro At first glance this looks great. But I don't see tests and handling for attributes which uses named arguments?

[nunomaduro]

@morloderex Thanks for bringing that to my attention. It's fixed.