Actually expose `crate::io::Error`

[rockboynton]

crate::io::Error is pub but crate::io is not pub so it is actually not exposed to the user. This makes it impossible to name in an application crate that wants to e.g. wrap that error

[8de2fdb0]

Yes it is impossible to use SerialPort<'a, _, _, _> as embedded_io::Write unless mod io becomes public.