Add Lwt_retry library

[shonfeder]

Adds the Lwt_retry library discussed in #1028 as a separate package. I believe all guidance suggested there by @raphael-proust has been incorporated. Thanks for that!

I did not add an iter function, since I think this doesn't add any value over using Lwt_stream.iter directly.

I've tried to make the code, documentation, and tests consistent with the style I found in other locations in the code base.