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/rustbook/main.rs

@@ -11,9 +11,9 @@
 #![feature(box_syntax)]
 #![feature(collections)]
 #![feature(core)]
-#![feature(io)]
+#![feature(old_io)]
 #![feature(os)]
-#![feature(path)]
+#![feature(old_path)]
 #![feature(rustdoc)]
 
 extern crate rustdoc;