Add `SimplexStream` structure

[wutchzone]

I think that it is unfortunate that the underlying structure backing the DuplexStream is not exposed. There are cases where a single direction will suffice, for example, when I have only a single writer and single reader, thus not requiring bidirectionality.

In this patch, I renamed the Pipe to the SimplexStream for coherence with the already existing DuplexStream and made it public