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/test/compile-fail/lint-uppercase-variables.rs

@@ -12,12 +12,14 @@
 
 #![allow(dead_code)]
 #![deny(non_snake_case)]
-#![feature(path)]
-#![feature(io)]
 
 use std::old_io::File;
 use std::old_io::IoError;
 
+mod foo {
+    pub enum Foo { Foo }
+}
+
 struct Something {
     X: usize //~ ERROR structure field `X` should have a snake case name such as `x`
 }
@@ -30,13 +32,10 @@ fn main() {
     let Test: usize = 0; //~ ERROR variable `Test` should have a snake case name such as `test`
     println!("{}", Test);
 
-    let mut f = File::open(&Path::new("something.txt"));
-    let mut buff = [0u8; 16];
-    match f.read(&mut buff) {
-        Ok(cnt) => println!("read this many bytes: {}", cnt),
-        Err(IoError{ kind: EndOfFile, .. }) => println!("Got end of file: {:?}", EndOfFile),
-//~^ ERROR variable `EndOfFile` should have a snake case name such as `end_of_file`
-//~^^ WARN `EndOfFile` is named the same as one of the variants of the type `std::old_io::IoErrorKind`
+    match foo::Foo::Foo {
+        Foo => {}
+//~^ ERROR variable `Foo` should have a snake case name such as `foo`
+//~^^ WARN `Foo` is named the same as one of the variants of the type `foo::Foo`
     }
 
     test(1);