wip: generic State<T>

closes #8

Zkusil jsem naťuknout generický State, ale ještě to není úúúúplně ono 🤔

<?php

final class Foo implements \Grifart\Stateful\Stateful
{

	public function _getState(): \Grifart\Stateful\State
	{
		return \Grifart\Stateful\StateBuilder::from($this)
			->version(1)
			->build();
	}

	public static function _fromState(\Grifart\Stateful\State $state): static
	{
		$obj = $state->makeAnEmptyObject(static::class);
		return $obj;
	}
}

vyprodukuje

------ -----------------------------------------------------------------------
  Line   test.php
 ------ -----------------------------------------------------------------------
  6      Return type (Grifart\Stateful\State<static(Foo)>) of method
         Foo::_getState() should be compatible with return type
         (Grifart\Stateful\State<static(Grifart\Stateful\Stateful)>) of method
         Grifart\Stateful\Stateful::_getState()
  8      Method Foo::_getState() should return
         Grifart\Stateful\State<static(Foo)> but returns
         Grifart\Stateful\State<Foo>.
  16     Method Foo::_fromState() should return static(Foo) but returns
         Grifart\Stateful\Stateful.
 ------ -----------------------------------------------------------------------