Better polyfill support for Stdlib and Session

This PR aims to provide more robust, simpler-to-use polyfill support for Zend\Stdlib and Zend\Session.

The current support requires that uses call on a `compatibility/autoload.php` file in order for the polyfill replacements to be made. This creates problems:

* Education. We have to educate those who use the component individually how to load that file, both with and without Composer.
* Breaks classmap generation. Since the same class is declared in multiple files, classmap generation in either Composer or ZF's own `classmap_generator.php` fails, as the incorrect variant may be used.

The proposed solution uses PHP's `class_alias`. This function allows you to globally alias one class name to another. I have put `class_alias` statements inside conditionals, achieving conditional imports:

```php
// In library/Zend/Stdlib/ArrayObject.php:

if (version_compare(PHP_VERSION, '5.4.0', 'lt')) {
    class_alias('Zend\Stdlib\ArrayObject\PhpLegacyCompatibility', 'Zend\Stdlib\AbstractArrayObject');
} else {
    class_alias('Zend\Stdlib\ArrayObject\PhpReferenceCompatibility', 'Zend\Stdlib\AbstractArrayObject');
}

class ArrayObject extends AbstractArrayObject
{
}
```

This approach means we no longer need to introduce autoloading hacks, and makes it possible for classmap generation to work properly again.