Fix get_metadata on windows

The MetadataExt functions are nightly-only right now so we need to call
the windows api ourselves to get the device number.  The winapi_util
package has a nice wrapper to do this for us.

The get_device function doesn't appear to be used anywhere so I removed
it.

Cargo.toml

@@ -26,6 +26,9 @@ clap = "=2.33"
 jwalk = "0.4.0"
 num_cpus = "1.12"
 
+[target.'cfg(windows)'.dependencies]
+winapi-util = "0.1"
+
 [dev-dependencies]
 assert_cli = "=0.6"
 tempfile = "=3"