std: Implement CString-related RFCs

This commit is an implementation of [RFC 592][r592] and [RFC 840][r840]. These
two RFCs tweak the behavior of `CString` and add a new `CStr` unsized slice type
to the module.

[r592]: https://github.com/rust-lang/rfcs/blob/master/text/0592-c-str-deref.md
[r840]: https://github.com/rust-lang/rfcs/blob/master/text/0840-no-panic-in-c-string.md

The new `CStr` type is only constructable via two methods:

1. By `deref`'ing from a `CString`
2. Unsafely via `CStr::from_ptr`

The purpose of `CStr` is to be an unsized type which is a thin pointer to a
`libc::c_char` (currently it is a fat pointer slice due to implementation
limitations). Strings from C can be safely represented with a `CStr` and an
appropriate lifetime as well. Consumers of `&CString` should now consume `&CStr`
instead to allow producers to pass in C-originating strings instead of just
Rust-allocated strings.

A new constructor was added to `CString`, `new`, which takes `T: IntoBytes`
instead of separate `from_slice` and `from_vec` methods (both have been
deprecated in favor of `new`). The `new` method returns a `Result` instead of
panicking.  The error variant contains the relevant information about where the
error happened and bytes (if present). Conversions are provided to the
`io::Error` and `old_io::IoError` types via the `FromError` trait which
translate to `InvalidInput`.

This is a breaking change due to the modification of existing `#[unstable]` APIs
and new deprecation, and more detailed information can be found in the two RFCs.
Notable breakage includes:

* All construction of `CString` now needs to use `new` and handle the outgoing
  `Result`.
* Usage of `CString` as a byte slice now explicitly needs a `.as_bytes()` call.
* The `as_slice*` methods have been removed in favor of just having the
  `as_bytes*` methods.

Closes #22469
Closes #22470
[breaking-change]

src/libstd/sys/unix/ext.rs

@@ -33,7 +33,7 @@
 
 use prelude::v1::*;
 
-use ffi::{CString, OsStr, OsString};
+use ffi::{CString, NulError, OsStr, OsString};
 use fs::{self, Permissions, OpenOptions};
 use net;
 use mem;
@@ -155,7 +155,7 @@ pub trait OsStrExt {
     fn as_bytes(&self) -> &[u8];
 
     /// Convert the `OsStr` slice into a `CString`.
-    fn to_cstring(&self) -> CString;
+    fn to_cstring(&self) -> Result<CString, NulError>;
 }
 
 impl OsStrExt for OsStr {
@@ -166,8 +166,8 @@ impl OsStrExt for OsStr {
         &self.as_inner().inner
     }
 
-    fn to_cstring(&self) -> CString {
-        CString::from_slice(self.as_bytes())
+    fn to_cstring(&self) -> Result<CString, NulError> {
+        CString::new(self.as_bytes())
     }
 }
 
@@ -249,5 +249,7 @@ impl ExitStatusExt for process::ExitStatus {
 /// Includes all extension traits, and some important type definitions.
 pub mod prelude {
     #[doc(no_inline)]
-    pub use super::{Fd, AsRawFd, OsStrExt, OsStringExt, PermissionsExt, CommandExt, ExitStatusExt};
+    pub use super::{Fd, AsRawFd, OsStrExt, OsStringExt, PermissionsExt};
+    #[doc(no_inline)]
+    pub use super::{CommandExt, ExitStatusExt};
 }