Invert <CString as Deref>::deref and CString::as_c_str

This is consistent with the style of `ByteString`.

Author: tamird

alloc/src/ffi/c_str.rs

@@ -574,7 +574,7 @@ impl CString {
     #[stable(feature = "as_c_str", since = "1.20.0")]
     #[rustc_diagnostic_item = "cstring_as_c_str"]
     pub fn as_c_str(&self) -> &CStr {
-        &*self
+        unsafe { CStr::from_bytes_with_nul_unchecked(self.as_bytes_with_nul()) }
     }
 
     /// Converts this `CString` into a boxed [`CStr`].
@@ -705,14 +705,14 @@ impl ops::Deref for CString {
 
     #[inline]
     fn deref(&self) -> &CStr {
-        unsafe { CStr::from_bytes_with_nul_unchecked(self.as_bytes_with_nul()) }
+        self.as_c_str()
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Debug for CString {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Debug::fmt(&**self, f)
+        fmt::Debug::fmt(self.as_c_str(), f)
     }
 }