Adds support of internal child class instantiation

[gquemener]

Related to #39

This is my attempt to resolve a long on-going issue on the phpspec repository, taking inspiration on the SF VarExporter component.

I do not see any other missing testcases, however feel free to let me know if you think about some, I will happily add them.

/cc @ciaranmcnulty @Ocramius @mikeSimonson

[gquemener]

Wait, don't merge yet, there's something odd going on

[gquemener]

Allright, that's better now. Cloning ArrayIterator results in the original array being referenced in cloned objects, which will probably lead to unwanted side effects.

Here's an example:

<?php

require __DIR__ . '/vendor/autoload.php';

final class MyIterator extends \ArrayIterator
{
}

$instantiator = new \Doctrine\Instantiator\Instantiator();
$o1 = $instantiator->instantiate(\MyIterator::class);
$o2 = $instantiator->instantiate(\MyIterator::class);
$o3 = $instantiator->instantiate(\MyIterator::class);
$o3->append('foo');
die(var_dump($o1, $o2, $o3));

Which outputs:

/app/try.php:14:
class MyIterator#2 (1) {
  private $storage =>
  array(1) {
    [0] =>
    string(3) "foo"
  }
}
/app/try.php:14:
class MyIterator#7 (1) {
  private $storage =>
  class MyIterator#6 (1) {
    private $storage =>
    class MyIterator#2 (1) {
      private $storage =>
      array(1) {
        [0] =>
        string(3) "foo"
      }
    }
  }
}
/app/try.php:14:
class MyIterator#8 (1) {
  private $storage =>
  class MyIterator#6 (1) {
    private $storage =>
    class MyIterator#2 (1) {
      private $storage =>
      array(1) {
        [0] =>
        string(3) "foo"
      }
    }
  }
}

[gquemener]

Let me know if you'd rather have one commit in this PR.

[Ocramius]

LGTM

[Ocramius]

Thanks @gquemener!