Stabilize core::ffi::CStr, alloc::ffi::CString, and friends

Stabilize the `core_c_str` and `alloc_c_string` feature gates.

Change `std::ffi` to re-export these types rather than creating type
aliases, since they now have matching stability.

library/std/src/lib.rs

@@ -264,7 +264,6 @@
 #![feature(atomic_mut_ptr)]
 #![feature(char_error_internals)]
 #![feature(char_internals)]
-#![feature(core_c_str)]
 #![feature(core_intrinsics)]
 #![feature(cstr_from_bytes_until_nul)]
 #![feature(cstr_internals)]
@@ -297,8 +296,6 @@
 //
 // Library features (alloc):
 #![feature(alloc_layout_extra)]
-#![feature(alloc_c_string)]
-#![feature(alloc_ffi)]
 #![feature(allocator_api)]
 #![feature(get_mut_unchecked)]
 #![feature(map_try_insert)]