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/compiletest/compiletest.rs

@@ -20,7 +20,6 @@
 #![feature(std_misc)]
 #![feature(test)]
 #![feature(core)]
-#![feature(path)]
 #![feature(io)]
 #![feature(net)]
 #![feature(path_ext)]

src/compiletest/procsrv.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![allow(deprecated)] // for old path, for dynamic_lib
+
 use std::process::{ExitStatus, Command, Child, Output, Stdio};
 use std::io::prelude::*;
 use std::dynamic_lib::DynamicLibrary;