Introduce the FSIterator.

First step consisted to remove `scandir` and replace it by
`FilesystemIterator` (see 8c81bc3). However, while it is a little bit
faster and cross-FS, it does not solve the problem of memory consumption
(and inherited CPU consumption) when having a lot of children.

Thus, this second step consists to solve this bottleneck. Instead of:

  1. iterating 1 time on children,
  2. transform them into nodes and
  3. return all the nodes as an array, which will be iterated a 2nd time,

we build an iterator that:

  1. given a collection (`ICollection`) and a path,
  2. generate and return a filesystem iterator that transforms children
     into nodes on demand.

Children are iterated later.

Bonus: a filter can be added, useful in `FSExt/Directory.php` for
instance.