ignore: publicize ignore::dir

[noahbkim]

I was wondering if any of the maintainers had a philosophical objection to making ignore::dir::Ignore and related machinery part of the public API. I've found myself needing to reimplement Walk with slightly modified file system abstractions, and it would be really helpful to be able to reuse the Ignore bits.

Happy to put up the PR if this seems reasonable.

[BayLee4]

This would be very useful especially with the ability to clone WalkBuilder.ig_builder, that would allow to check if a currently non-existing file/directory would be ignored before creating it.