.with_c_str() is a replacement for the old .as_c_str(), to avoid
unnecessary boilerplate.
Replace all usages of .to_c_str().with_ref() with .with_c_str().
Previous dicussions about CString suggested that interior nulls should
throw an error. This was never implemented. Add this now, using a
condition (named null_byte) to allow for recovery.
Add method .to_c_str_unchecked() that skips this check.