ignore: add 'same_file_system' option

This commit adds a 'same_file_system' option to the walk builder. For
single threaded walking, it defers to the walkdir crate, which has the
same option. The bulk of this commit implements this flag for the parallel
walker. We add one very feeble test for this.

The parallel walker is now officially a complete mess.

Closes #321

ignore/src/lib.rs

@@ -60,7 +60,7 @@ extern crate tempdir;
 extern crate thread_local;
 extern crate walkdir;
 #[cfg(windows)]
-extern crate winapi;
+extern crate winapi_util;
 
 use std::error;
 use std::fmt;