Use `as_bytes_with_nul` for deepsize of `CString`

[lopopolo]

CString::as_bytes_with_nul is cheap to call - it directly returns a reference to the inner byte slice.

as_bytes_with_nul performs no allocations or validations.

See:

• https://doc.rust-lang.org/std/ffi/struct.CString.html#method.as_bytes_with_nul
• https://doc.rust-lang.org/src/std/ffi/c_str.rs.html#635-637

as_bytes_with_nul has been stable since Rust 1.0.0.