[Debug] cleanup interfaces before 2.5-final
[Debug] better ouf of memory error handling
feature #10941 [Debug] cleanup interfaces before 2.5-final (nicolas-grekas)

This PR was merged into the 2.4-dev branch.

Discussion
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[Debug] cleanup interfaces before 2.5-final

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

This PR is targeted at cleaning up interfaces before 2.5 final:
- ExceptionHandlerInterface has never been released in a stable Symfony, lets drop it, not deprecate it,
- generalize a little bit how fatal errors are handled and make them take the same path as uncaught exceptions,
- enhance handling of out of memory situations.

Commits
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e3255bf [Debug] better ouf of memory error handling
dfa8ff8 [Debug] cleanup interfaces before 2.5-final