msgpack serialization of interfaces

[vkaverin]

_main();

function _main() {
  /** @var I $i */
  $i = new A(42);
  instance_serialize($i);
}

interface I {}

/** @kphp-serializable */
class A implements I{

  /** @kphp-serialized-field 1 */
  private int $x;

  public function __construct(int $x) {
    $this->x = $x;
  }
}

This code's compilation fails with

Compilation error at stage: Apply phpdocs, gen by parse-and-apply-phpdoc.cpp:677
  kphp_script_dummy.php:17
    interface I {}

@kphp-serialize is allowed only for classes

Serialization of interfaces if forbidden, because serialization doesn't store any class-ID of the serialized object and so can not be deserialized.

Probably for interfaces we can allow some overhead and store class name in serialization result too.