Add new path module

Implements [RFC 474](rust-lang/rfcs#474); see
that RFC for details/motivation for this change.

This initial commit does not include additional normalization or
platform-specific path extensions. These will be done in follow up
commits or PRs.

Author: aturon

src/libstd/ffi/mod.rs

@@ -24,6 +24,7 @@ pub use self::os_str::OsStr;
 mod c_str;
 mod os_str;
 
+// FIXME (#21670): these should be defined in the os_str module
 /// Freely convertible to an `&OsStr` slice.
 pub trait AsOsStr {
     /// Convert to an `&OsStr` slice.

src/libstd/ffi/os_str.rs

@@ -41,7 +41,7 @@ use string::{String, CowString};
 use ops;
 use cmp;
 use hash::{Hash, Hasher, Writer};
-use path::{Path, GenericPath};
+use old_path::{Path, GenericPath};
 
 use sys::os_str::{Buf, Slice};
 use sys_common::{AsInner, IntoInner, FromInner};