std: Add a new fs module

This commit is an implementation of [RFC 739][rfc] which adds a new `std::fs`
module to the standard library. This module provides much of the same
functionality as `std::old_io::fs` but it has many tweaked APIs as well as uses
the new `std::path` module.

[rfc]: https://github.com/rust-lang/rfcs/pull/739

src/libstd/path.rs

@@ -999,6 +999,12 @@ impl cmp::Ord for PathBuf {
     }
 }
 
+impl AsOsStr for PathBuf {
+    fn as_os_str(&self) -> &OsStr {
+        &self.inner[]
+    }
+}
+
 /// A slice of a path (akin to `str`).
 ///
 /// This type supports a number of operations for inspecting a path, including