[VarDumper] enhance dumping refs

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | nicolas-grekas/Patchwork-Dumper#39
| License       | MIT
| Doc PR        | -

This makes dumps prettier:
- VarCloner now exposes the "isref" state of a PHP variable
- on hard refs, `#n` suffix is replaced by `&n` prefix
- `#n`/`@n` (object/resource refs) are moved inside the `{ }`
- all repetitive dumps of the same object are toggle-able
- all repetitive dumps of the same object / hard ref are *visually* identical
- dumps expose local ref counts and object's handle

I also simplified the `Cloner\DumperInterface`.

For the same (circumvoluted) variable:

Before:
![before](https://cloud.githubusercontent.com/assets/243674/4595026/f6b2a98c-5094-11e4-9fe0-dc6d24da3f14.png)

After:
![after](https://cloud.githubusercontent.com/assets/243674/4595035/07e04980-5095-11e4-9e45-54a1e7453547.png)