slow on CephFS, introduce parallel walk #579

The current implementation uses Walk from the ignore crate from ripgrep.

However, this is very slow on some file systems, such as CephFS with many directories and files.
I see that there is also WalkParallel:
https://github.com/BurntSushi/ripgrep/blob/3fce3b5bb0236da2df6d99672afb8a719642eca7/crates/ignore/src/walk.rs#L701

Could Rustic easily switch to it?
It does not implement Iterator, so it'd require some transform at least.