[Debug] Symfony debug extension

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

Add a `symfony_zval_info(&$var, $options = 0)` function in a PHP extension written in C, that:

- exposes zval_hash/refcount, allowing efficient exploration of arbitrary structure in PHP,
- does work with references, preventing memory copying.