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]

1 files changed, 1 insertions, 0 deletions

diff --git a/src/libstd/ffi/os_str.rs b/src/libstd/ffi/os_str.rs
index 77df831bbfe..feacbf1e98b 100644
--- a/src/libstd/ffi/os_str.rs
+++ b/src/libstd/ffi/os_str.rs
@@ -346,6 +346,7 @@ impl AsOsStr for String {
     }
 }
 
+#[allow(deprecated)]
 impl AsOsStr for Path {
     #[cfg(unix)]
     fn as_os_str(&self) -> &OsStr {