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/libstd/old_path/mod.rs

@@ -59,7 +59,7 @@
 //! println!("path exists: {}", path.exists());
 //! ```
 
-#![unstable(feature = "path")]
+#![unstable(feature = "old_path")]
 
 use core::marker::Sized;
 use ffi::CString;