Port ls to ftw

[JRF63]

Had to modify ftw quite a bit to accommodate the required directory listing order for ls. This PR modified more files than necessary because of the need to add new options to ftw::traverse_directory - I used the struct update syntax to hopefully avoid needing to do it again in the future if more options are added:

        ftw::TraverseDirectoryOpts {
            list_contents_first: true,
            ..Default::default() // Sets the other options to their default values
        },

Also fixed clippy errors on the modified files while I was at it.