[Debug] error stacking + fatal screaming + case testing

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | no
| License       | MIT
| Doc PR        | no

Ported from https://github.com/nicolas-grekas/Patchwork/

Three enhancements for Symfony debug mode:

- detect case mismatches between loaded class name, its declared name and its source file name
- dismiss hard to debug blank pages related to non-catchable-@-silenced fatal errors (```@(new Toto) + parse error in Toto.php``` => enjoy debugging)
- work around https://bugs.php.net/42098 / https://bugs.php.net/54054 / https://bugs.php.net/60149 / https://bugs.php.net/65322 (fixed in 5.5.5)

An other thing I didn't port is scope isolation: the current `require` in autoloaders is done in their scope, so local $vars / $this (with access to private props/methods) is easily accessible from required files. Shouldn't proper separation prevent that?