Replace deprecated tempdir dependency with tempfile (#11588)

`Tempdir` is deprecated, but the functionality has been merged into
another crate: `tempfile`. This commit removes all `tempdir` dependencies
and replaces them with `tempfile` and the equivalent bindings.

Fixes #11560

ethcore/light/src/net/load_timer.rs

@@ -264,7 +264,7 @@ mod tests {
 
 	#[test]
 	fn file_store() {
-		let tempdir = ::tempdir::TempDir::new("").unwrap();
+		let tempdir = ::tempfile::TempDir::new().unwrap();
 		let path = tempdir.path().join("file");
 		let store = FileStore(path);