Add generic conversion traits

This commit:

* Introduces `std::convert`, providing an implementation of
RFC 529.

* Deprecates the `AsPath`, `AsOsStr`, and `IntoBytes` traits, all
in favor of the corresponding generic conversion traits.

  Consequently, various IO APIs now take `AsRef<Path>` rather than
`AsPath`, and so on. Since the types provided by `std` implement both
traits, this should cause relatively little breakage.

* Deprecates many `from_foo` constructors in favor of `from`.

* Changes `PathBuf::new` to take no argument (creating an empty buffer,
  as per convention). The previous behavior is now available as
  `PathBuf::from`.

* De-stabilizes `IntoCow`. It's not clear whether we need this separate trait.

Closes #22751
Closes #14433

[breaking-change]

1 files changed, 2 insertions, 0 deletions

diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index 9f217bba00a..9af7b9ab633 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -39,6 +39,8 @@
 #![feature(unicode)]
 #![feature(path_ext)]
 #![feature(str_char)]
+#![feature(convert)]
+#![feature(into_cow)]
 
 extern crate arena;
 extern crate fmt_macros;