Backport `Either` to 0.4

[daxpedda]

With the latest changes, moving types to http-body-util, I think this will require a breaking change release for a lot of crates.

I have a use-case where I require the recently introduced Either type from #64, currently I basically just copied the file. Would it be possible to backport #64 to a 0.4 branch and make a release?

Happy to make the PR of course.