UPGRADE FROM 2.7.1 to 2.7.2

Form

* In order to fix a few regressions in the new `ChoiceList` implementation,

a few details had to be changed compared to 2.7.

The legacy `Symfony\Component\Form\Extension\Core\ChoiceList\ChoiceListInterface`

now does not extend the new `Symfony\Component\Form\ChoiceList\ChoiceListInterface`

anymore. If you pass an implementation of the old interface in a context

where the new interface is required, wrap the list into a

`LegacyChoiceListAdapter`:

Before:

```php

use Symfony\Component\Form\ChoiceList\ChoiceListInterface;

function doSomething(ChoiceListInterface $choiceList)

{

// ...

}

doSomething($legacyList);

```

After:

```php

use Symfony\Component\Form\ChoiceList\ChoiceListInterface;

use Symfony\Component\Form\ChoiceList\LegacyChoiceListAdapter;

function doSomething(ChoiceListInterface $choiceList)

{

// ...

}

doSomething(new LegacyChoiceListAdapter($legacyList));

```

The new `ChoiceListInterface` now has two additional methods

`getStructuredValues()` and `getOriginalKeys()`. You should add these methods

if you implement this interface. See their doc blocks and the implementation

of the core choice lists for inspiration.

The method `ArrayKeyChoiceList::toArrayKey()` was marked as internal. This

method was never supposed to be used outside the class.

The method `ChoiceListFactoryInterface::createView()` does not accept arrays

and `Traversable` instances anymore for the `$groupBy` parameter. Pass a

callable instead.