std: Redesign c_str and c_vec

This commit is an implementation of [RFC 494][rfc] which removes the entire
`std::c_vec` module and redesigns the `std::c_str` module as `std::ffi`.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0494-c_str-and-c_vec-stability.md

The interface of the new `CString` is outlined in the linked RFC, the primary
changes being:

* The `ToCStr` trait is gone, meaning the `with_c_str` and `to_c_str` methods
  are now gone. These two methods are replaced with a `CString::from_slice`
  method.
* The `CString` type is now just a wrapper around `Vec<u8>` with a static
  guarantee that there is a trailing nul byte with no internal nul bytes. This
  means that `CString` now implements `Deref<Target = [c_char]>`, which is where
  it gains most of its methods from. A few helper methods are added to acquire a
  slice of `u8` instead of `c_char`, as well as including a slice with the
  trailing nul byte if necessary.
* All usage of non-owned `CString` values is now done via two functions inside
  of `std::ffi`, called `c_str_to_bytes` and `c_str_to_bytes_with_nul`. These
  functions are now the one method used to convert a `*const c_char` to a Rust
  slice of `u8`.

Many more details, including newly deprecated methods, can be found linked in
the RFC. This is a:

[breaking-change]
Closes #20444

1 files changed, 3 insertions, 3 deletions

diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 608ad9882b9..2d3a4639379 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -208,10 +208,10 @@ pub mod num;
 
 /* Runtime and platform support */
 
-pub mod thread_local;
-pub mod c_str;
-pub mod c_vec;
+pub mod thread_local; // first for macros
+
 pub mod dynamic_lib;
+pub mod ffi;
 pub mod fmt;
 pub mod io;
 pub mod os;