move tuf::client::PathTranslator to FileSystemRepository

[erickt]

tuf::client::PathTranslator allows clients to use an alternative path separator, but according to theupdateframework/specification#63 (comment), the targets metadata path separators should always be /. So it makes more sense that the PathTranslator be a property on tuf::repository::FileSystemRepository, since it's the only thing that really needs to map target paths to files on a file system.

[heartsucker]

At that point it would be easier to just strip it out entirely and then do a split on / and replace it with \\ for windows. No need for a whole trait.