Add to_path_normalized

[matklad]

I have a ../foo RelativePathBuf and a /bar std::PathBuf. I'd like to be able to get /foo out of it, without calling Path::canonicalize (which is a failable syscall). Perhaps adding a to_path_normalized with this semantics would work?

[udoprog]

Mentoring notes:

This would require implementing a version of relative_traversal that works with a mix of path::Component and relative_path::Component over a PathBuf instance.

Anyone who is interested in normalization might also want to go and shout in rust-lang/rust#47402 since that would supersede the need to do it in this crate.

[udoprog]

I think we already support this with RelativePath::normalize now.