std: Rename io/path features with old_ prefix

This commit renames the features for the `std::old_io` and `std::old_path`
modules to `old_io` and `old_path` to help facilitate migration to the new APIs.

This is a breaking change as crates which mention the old feature names now need
to be renamed to use the new feature names.

[breaking-change]

src/librustc_llvm/lib.rs

@@ -29,7 +29,7 @@
 #![feature(int_uint)]
 #![feature(libc)]
 #![feature(link_args)]
-#![feature(path)]
+#![feature(old_path)]
 #![feature(staged_api)]
 #![feature(std_misc)]