Add the ability to cast an object to mixed type

[mkornaukhov03]

<?php
class A { }

function foo(mixed $x) {
	if ($x instanceof A) {
		echo "A" . "\n";
	}
	else {
		echo "Not A" . "\n";
	}
}

foo(new A);
foo(42);

For now, KPHP shows the following error:

Compilation error at stage: Calc actual calls, gen by type-inferer.cpp:53
  index.php:5  in foo
    if ($x instanceof A) {

pass mixed to argument $instance of instance_cast
but it's declared as @param object

  index.php:5  in foo
    if ($x instanceof A) {
    $x is mixed

  index.php:4  in foo
    function foo(mixed $x) {
    argument $x declared as mixed

This functionality assumes two tasks:

1. Track all the places where object is cast to mixed
2. Change the mixed implementation

Talking about mixed implementation, we should add OBJECT value to mixed::type.
In layout: mixed.storage should contain a pointer to an object.
Key note: for all classes, that are cast to mixed, we should generate the following code(A class from previous snippet:

struct VirtualBaseForMixedClass { virtual ~VirtualBaseForMixedClass() = default; };
...
struct C$A : VirtualBaseForMixedClass { ~C$A() {} };

It will allow us to do the following:

$x instanceof A

--> (pseudocode)

auto* $tmp = dynamic_cast<rhs.as_instanceof()::class_type *>(reinterpret_cast<VirtualBaseForMixedClass*>(lhs.as_mixed().storage))
if ($tmp == nullptr) { error(...) }

[vkaverin]

But why? The fact that a class instance cannot be mixed was a fundamental statement for KPHP for years, why break this rule now?

[mkornaukhov03]

But why? The fact that a class instance cannot be mixed was a fundamental statement for KPHP for years, why break this rule now?

It allows a bigger subset of PHP to be compiled. But for now it's only idea about its implementation in runtime, not about type system.