[Debug] expose object_handle

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This is a small enhancement to the symfony debug C extension that allows fetching an object's handle.
This is the `#number` as displayed by var_dump():
`class stdClass**#1** (0) {}`

This is required for VarDumper to be able to expose objects' handles and thus have more precise dumps.
It will allow inspecting "same" relationships between two *different* dumps.