This PR was merged into the 5.1-dev branch.

Discussion
----------

[DI] add syntax to stack decorators

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #30599
| License       | MIT
| Doc PR        | -

Declare this:
```yaml
services:
    my_stack_of_decorators:
        stack:
            - class: App\ExternalDecorator
            - class: App\InternalDecorator
            - class: App\DecoratoredClass
```

And get this:
![image](https://user-images.githubusercontent.com/243674/78615803-b8c8e580-7872-11ea-95c2-22cb78f88ca8.png)

The PR is now ready with support for Yaml, XML and the PHP-DSL. It needs #36388, #36392 and #36389 to pass, and relates to #36390 to be DX-friendly.

The new syntax now supports composable stacks - i.e stack you can reuse in the middle of another stack.

RIP middleware, simple decorators FTW :)

From the test cases:
```yaml
services:
    reusable_stack:
        stack:
            - class: stdClass
              properties:
                  label: A
                  inner: '@.inner'
            - class: stdClass
              properties:
                  label: B
                  inner: '@.inner'

    concrete_stack:
        stack:
            - parent: reusable_stack
            - class: stdClass
              properties:
                  label: C
```

This will create a service similar to:
```php
(object) [
    'label' => 'A',
    'inner' => (object) [
        'label' => 'B',
        'inner' => (object) [
             'label' => 'C',
        ]
    ],
];
```

When used together with autowiring, this is enough to declare a stack of decorators:
```yaml
services:
    my_processing_stack:
        stack:
            - App\ExternalDecorator: ~
            - App\InternalDecorator: ~
            - App\TheDecoratedClass: ~
```

See fixtures for the other configuration formats.

See also https://twitter.com/nicolasgrekas/status/1248198573998604288

Todo:
- [x] rebase on top of #36388, #36392 and #36389 once they are merged
- [x] test declaring deeper nested stacks

Commits
-------

98eeeae [DI] add syntax to stack decorators