This adds a member fn that converts a slice into a CStr; it is intended
to be safer than from_ptr (which is unsafe and may read out of bounds),
and more useful than from_bytes_with_nul (which requires that the caller
already know where the nul byte is).
feature gate: cstr_from_bytes_until_nul
Also add an error type FromBytesUntilNulError for this fn.