Add `Paths::files` and `Paths::dirs`

[vitiral]

This issue proposes adding the following methods:

• Paths::files(self) -> Files: returns an iterator of only file types.
• Paths::dirs(self) -> Dirs: returns an iterator of only directory types.

In both cases, symlinks are resolved as either files or dirs and returned as the original paths.

If you approve I can implement this.

[NicTanghe]

I`m a bit confuzed on how ergo-fs whoud fix this, does it work on windows ?

[vitiral]

I... don't remember. I'm not actively working on rust any longer. Feel free to close.