[POC] added scanner adapter

This PR is a POC trying to improve finder performance using a pure PHP adapter.

This adapter:
- only improve tests against the filename/path
- don't visit children of excluded directoris
- tries to make the more calculations outside the loops

Benchmark:
- result can be found here: https://gist.github.com/jfsimon/f58d6dda383be9f1bfdd
- used framework here: https://github.com/jfsimon/symfony-finder-benchmark

I grouped all new classes in a single `Scanner` namespace.