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/lib.rs

@@ -251,6 +251,7 @@ pub mod old_io;
 pub mod os;
 pub mod env;
 pub mod path;
+pub mod old_path;
 pub mod rand;
 pub mod time;