[Debug] Improvements, compat and Object tracer

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Here are some ideas that we worked on with @jpauli and that we propose for enhancement and inclusion in the symfony_debug extension for 2.7:
- symfony_debug_backtrace() is like debug_backtrace(), except that it can fetch the full backtrace in case of fatal errors (prod friendly version of xdebug_get_function_stack())
- symfony_debug_set_logger() logs objects' creations/destructions

Your ideas are more than welcomed: the name/interface of the functions can certainly be improved, and if you have more ideas, we can't wait to hear them :)