decoder: aio: Make `TokioReader` public

decoder: aio: Make `TokioReader` public

This exposes `decoder::aio::TokioReader<T>` in a similar manner to
`decoder::sync::StandardReader<T>`, which is necessary if one wants
to remain generic over `T: tokio::io::AsyncRead`, e.g.:

> struct FooDecoder<T: tokio::io::AsyncRead> {
>    decoder: aio::Decoder<aio::TokioReader<T>>,
> }

Signed-off-by: Max Carrara <m.carrara@proxmox.com>