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/old_io/process.rs

@@ -204,7 +204,7 @@ impl Command {
     /// otherwise configure the process.
     pub fn new<T: BytesContainer>(program: T) -> Command {
         Command {
-            program: CString::from_slice(program.container_as_bytes()),
+            program: CString::new(program.container_as_bytes()).unwrap(),
             args: Vec::new(),
             env: None,
             cwd: None,
@@ -219,14 +219,14 @@ impl Command {
 
     /// Add an argument to pass to the program.
     pub fn arg<'a, T: BytesContainer>(&'a mut self, arg: T) -> &'a mut Command {
-        self.args.push(CString::from_slice(arg.container_as_bytes()));
+        self.args.push(CString::new(arg.container_as_bytes()).unwrap());
         self
     }
 
     /// Add multiple arguments to pass to the program.
     pub fn args<'a, T: BytesContainer>(&'a mut self, args: &[T]) -> &'a mut Command {
         self.args.extend(args.iter().map(|arg| {
-            CString::from_slice(arg.container_as_bytes())
+            CString::new(arg.container_as_bytes()).unwrap()
         }));
         self
     }
@@ -239,8 +239,8 @@ impl Command {
                 // if the env is currently just inheriting from the parent's,
                 // materialize the parent's env into a hashtable.
                 self.env = Some(os::env_as_bytes().into_iter().map(|(k, v)| {
-                    (EnvKey(CString::from_slice(&k)),
-                     CString::from_slice(&v))
+                    (EnvKey(CString::new(k).unwrap()),
+                     CString::new(v).unwrap())
                 }).collect());
                 self.env.as_mut().unwrap()
             }
@@ -254,8 +254,8 @@ impl Command {
     pub fn env<'a, T, U>(&'a mut self, key: T, val: U)
                          -> &'a mut Command
                          where T: BytesContainer, U: BytesContainer {
-        let key = EnvKey(CString::from_slice(key.container_as_bytes()));
-        let val = CString::from_slice(val.container_as_bytes());
+        let key = EnvKey(CString::new(key.container_as_bytes()).unwrap());
+        let val = CString::new(val.container_as_bytes()).unwrap();
         self.get_env_map().insert(key, val);
         self
     }
@@ -263,7 +263,7 @@ impl Command {
     /// Removes an environment variable mapping.
     pub fn env_remove<'a, T>(&'a mut self, key: T) -> &'a mut Command
                              where T: BytesContainer {
-        let key = EnvKey(CString::from_slice(key.container_as_bytes()));
+        let key = EnvKey(CString::new(key.container_as_bytes()).unwrap());
         self.get_env_map().remove(&key);
         self
     }
@@ -276,15 +276,15 @@ impl Command {
                                  -> &'a mut Command
                                  where T: BytesContainer, U: BytesContainer {
         self.env = Some(env.iter().map(|&(ref k, ref v)| {
-            (EnvKey(CString::from_slice(k.container_as_bytes())),
-             CString::from_slice(v.container_as_bytes()))
+            (EnvKey(CString::new(k.container_as_bytes()).unwrap()),
+             CString::new(v.container_as_bytes()).unwrap())
         }).collect());
         self
     }
 
     /// Set the working directory for the child process.
     pub fn cwd<'a>(&'a mut self, dir: &Path) -> &'a mut Command {
-        self.cwd = Some(CString::from_slice(dir.as_vec()));
+        self.cwd = Some(CString::new(dir.as_vec()).unwrap());
         self
     }
 
@@ -1226,7 +1226,7 @@ mod tests {
         cmd.env("path", "foo");
         cmd.env("Path", "bar");
         let env = &cmd.env.unwrap();
-        let val = env.get(&EnvKey(CString::from_slice(b"PATH")));
-        assert!(val.unwrap() == &CString::from_slice(b"bar"));
+        let val = env.get(&EnvKey(CString::new(b"PATH").unwrap()));
+        assert!(val.unwrap() == &CString::new(b"bar").unwrap());
     }
 }