Lwt core refactored and documented to be contributor-friendly

The legible, human-friendly Lwt core is, I believe, just about ready for merge. If anyone wants to/has time to sneak in some final reviews, please glance at the PR :slight_smile: Its current status is summarized in a recent comment. I hope to merge this thing on Sunday or Monday; stop me if that’s wrong!!

Interestingly, this PR was held up by the Skylake CPU bug. With all the (formerly) undiagnosed segfaults, we couldn’t really tell if the old Lwt and new Lwt were failing in the same way, and therefore had no confidence to merge such a heavy refactoring.

Thanks again to all the persons involved in troubleshooting the Skylake bug for unblocking this! Thanks to all the reviewers.

(P.S. With reference to the video posted by @vramana above, I watched it – thanks again to @vramana; as a result, among other things, Lwt now has a thriving easy issues list).