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/liballoc/lib.rs

@@ -73,6 +73,7 @@
 #![feature(unboxed_closures)]
 #![feature(unsafe_no_drop_flag)]
 #![feature(core)]
+#![feature(unique)]
 #![cfg_attr(test, feature(test, alloc, rustc_private))]
 #![cfg_attr(all(not(feature = "external_funcs"), not(feature = "external_crate")),
             feature(libc))]