Stabilize std::path

This commit stabilizes essentially all of the new `std::path` API. The
API itself is changed in a couple of ways (which brings it in closer
alignment with the RFC):

* `.` components are now normalized away, unless they appear at the
  start of a path. This in turn effects the semantics of e.g. asking for
  the file name of `foo/` or `foo/.`, both of which yield `Some("foo")`
  now. This semantics is what the original RFC specified, and is also
  desirable given early experience rolling out the new API.

* The `parent` function now succeeds if, and only if, the path has at
  least one non-root/prefix component. This change affects `pop` as
  well.

* The `Prefix` component now involves a separate `PrefixComponent`
  struct, to better allow for keeping both parsed and unparsed prefix data.

In addition, the `old_path` module is now deprecated.

Closes #23264

[breaking-change]

src/libsyntax/parse/token.rs

@@ -25,6 +25,7 @@ use serialize::{Decodable, Decoder, Encodable, Encoder};
 use std::fmt;
 use std::mem;
 use std::ops::Deref;
+#[allow(deprecated)]
 use std::old_path::BytesContainer;
 use std::rc::Rc;
 
@@ -638,6 +639,7 @@ impl Deref for InternedString {
     fn deref(&self) -> &str { &*self.string }
 }
 
+#[allow(deprecated)]
 impl BytesContainer for InternedString {
     fn container_as_bytes<'a>(&'a self) -> &'a [u8] {
         // FIXME #12938: This is a workaround for the incorrect signature