…t. (dunglas)

This PR was merged into the 2.7 branch.

Discussion
----------

[Serializer] Refactoring and object_to_populate support.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | n/a

This PR is mainly a refactoring. It move up the common code of `PorpertyNormalizer` and `GetSetMethodNormalizer` into `AbstractNormalizer`.

It also adds a news feature: the ability to use an existing object instance for denormalization:

```php
$dummy = new GetSetDummy();
$dummy->setFoo('foo');

$obj = $this->normalizer->denormalize(
    array('bar' => 'bar'),
    'GetSetDummy',
     null,
     array('object_to_populate' => $dummy)
);

// $obj and $dummy are references to the same object
```

Commits
-------

b5990be [Serializer] Refactoring and object_to_populate support.

This PR was merged into the 2.7 branch.

Discussion
----------

[Serializer] ObjectNormalizer

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR       | not yet

`PropertyAccessNormalizer` is a new normalizer leveraging the PropertyAccess Component. It is able to handle classes containing both public properties and properties only accessibles trough getters / setters / issers / hassers...

As it extends `AbstractNormalizer`, it supports circular reference handling, name converters and existing object population.
What do you think about making this new normalizer the default one as it's the most convenient to use and the most consistent with the behavior of other components.

#13120, #13252 and #13255 need to be merged to make this PR working.

Commits
-------

0050bbb [Serializer] Introduce ObjectNormalizer