…service by keeping a reference to the old one (romainneutron)

This PR was merged into the 2.5-dev branch.

Discussion
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[DependencyInjection] added a simple way to replace a service by keeping a reference to the old one

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #5920
| License       | MIT
| Doc PR | symfony/symfony-docs#3745

This PR replaces #9003. Here's the todo list:
 - [x] add a cookbook entry for this new feature
 - [x] add support in all loaders/dumpers
 - [x] add unit tests
 - [x] see if there are use cases in Symfony that would benefit from this new feature
 - [x] find the best name for this feature

I've implemented YAML and XML Loader / Dumper.
From what I see, PhpDumper, PhpLoader, IniLoader, GraphvizDumper do not require an update, am I wrong?

Commits
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140f807 [DependencyInjection] Update dumpers and loaders, add unit tests
1eb1f4d [DependencyInjection] added a simple way to replace a service by keeping a reference to the old one

…ecoration (romainneutron)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #3753).

Discussion
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[DependencyInjection] Add documentation about service decoration

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes (symfony/symfony#10600)
| Applies to    | 2.5+
| Fixed tickets | #3745

Commits
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578d2e2 [DependencyInjection] Add documentation about service decoration