…ault)

This PR was squashed before being merged into the 5.2-dev branch.

Discussion
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[Serializer] Add CompiledClassMetadataFactory

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ?
| License       | MIT
| Doc PR        | todo (issue: symfony/symfony-docs#10706)

This introduce a dumped `ClassMetadataFactoryInterface` implementation to speed up the serializer by leveraging [PHP7 immutable array](https://blog.blackfire.io/php-7-performance-improvements-immutable-arrays.html).

Like for #28865, if the user have the opcache extension enabled, the compilation time will be skipped. The user will also have a performance boost when not using opcache as we are no longer fetching `ClassMetadata` from the PSR-6 cache.

This allow to speed up the normalization (without opcache) by 9-12% depending on how many objects are involved in the graph:

- [SymfonyObjectNormalizerBenchmark, 100 iterations with complexity of 1](https://blackfire.io/profiles/compare/d937a9cc-eebf-47eb-be90-c8e65cdf12b3/graph)
- [SymfonyObjectNormalizerBenchmark, 3 iterations with complexity of 60](https://blackfire.io/profiles/compare/d490542c-9a79-48a0-b7bc-1ed3ca6a9148/graph)

On the `FrameworkBundle` side, I suggest to add a `CacheWarmer` to dump the metadata array from configured class list. The list could have a _good_ default which will load the classes found in `src/Entity`.

## Dumping the `ClassMetadata`
```php
$classMetadatas = [];

foreach([Category::class, Comment::class, Forum::class, Thread::class, User::class] as $class) {
    $classMetadatas[] = $this->classMetadataFactory->getMetadataFor($class);
}

file_put_contents('dumped.php', $this->classMetadataFactoryCompiler->compile($classMetadatas));
```

## Using the dumped `ClassMetadata`
```php
$classMetadataFactory = new CompiledClassMetadataFactory(
    'dumped.php',
    new CacheClassMetadataFactory(
        new ClassMetadataFactory(new AnnotationLoader(new AnnotationReader())),
        new ApcuAdapter('SymfonyMetadata')
    )
);
```
# To do

- [x] Tests.
- [x] Cache warmer.
- [x] Documentation.
- [x] Changelog entry.

Commits
-------

63cbf0a [Serializer] Add CompiledClassMetadataFactory