Rename std::path to std::old_path

As part of [RFC 474](https://github.com/rust-lang/rfcs/pull/474), this
commit renames `std::path` to `std::old_path`, leaving the existing path
API in place to ease migration to the new one. Updating should be as
simple as adjusting imports, and the prelude still maps to the old path
APIs for now.

[breaking-change]

src/libstd/old_io/net/pipe.rs

@@ -23,7 +23,7 @@
 use prelude::v1::*;
 
 use ffi::CString;
-use path::BytesContainer;
+use old_path::BytesContainer;
 use old_io::{Listener, Acceptor, IoResult, TimedOut, standard_error};
 use sys::pipe::UnixAcceptor as UnixAcceptorImp;
 use sys::pipe::UnixListener as UnixListenerImp;