std: Deprecate std::old_io::fs

This commit deprecates the majority of std::old_io::fs in favor of std::fs and
its new functionality. Some functions remain non-deprecated but are now behind a
feature gate called `old_fs`. These functions will be deprecated once
suitable replacements have been implemented.

The compiler has been migrated to new `std::fs` and `std::path` APIs where
appropriate as part of this change.

src/libserialize/lib.rs

@@ -35,6 +35,7 @@ Core encoding and decoding interfaces.
 #![feature(staged_api)]
 #![feature(std_misc)]
 #![feature(unicode)]
+#![feature(path)]
 #![cfg_attr(test, feature(test))]
 
 // test harness access