Function start 

pub async fn start(
    kind: FsOpKind,
    src: End,
    sources: Vec<String>,
    dest: End,
    dest_dir: String,
    emit: Arc<dyn Fn(&FsOpEvent) + Send + Sync>,
) -> Result<FsOpStartResult, String>

Start a copy or move of sources (on src) into dest_dir (on dest) as an fs.op (emit carries its events). At least one side is a host.