[WIP] var-dumper component

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.6
| Fixed tickets | none

- [x] general introduction
- [x] cloner doc
- [x] caster doc
- [ ] dumper doc
- [ ] example output
- [ ] enhance the RST (with others, I'm far from a specialist)
I suggest ``filter out Doctrine internals``