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/librustc_llvm/lib.rs

@@ -28,9 +28,9 @@
 #![feature(int_uint)]
 #![feature(libc)]
 #![feature(link_args)]
-#![feature(old_path)]
 #![feature(staged_api)]
 #![feature(std_misc)]
+#![feature(path)]
 
 extern crate libc;
 #[macro_use] #[no_link] extern crate rustc_bitflags;