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.

[raphael-proust]

Thanks! I'll have a look at the dune version thing this week

[shonfeder]

Rebased on #1035 so I could update the package author and maintainer data. I'll need to rebase on master once that is merged in.

The only CI failures remain the FreeBSD and MacOS failures that are occurring in the trunk:

Test 'getgrgid and Unix.getgrgid' in suite 'lwt_unix' raised 'Not_found'
Test 'getgrnam and Unix.getgrnam' in suite 'lwt_unix' raised 'Not_found'

[raphael-proust]

I'll need to rebase on master once that is merged in

you can go ahead @shonfeder

[shonfeder]

Rebased! Should be good for a final review and/or merge! Thanks again for the reviews and guidance along the way, @raphael-proust :)

[shonfeder]

Anything else you'd like to see here @raphael-proust ?

[raphael-proust]

Ready to merge unless the CI shows I messaed up something with my suggested+applied changes

[shonfeder]

Thank you for the fixes!

[raphael-proust]

Thanks for the contribution!!