Port to Eio and Cohttp

[aantron]

This is a messy draft port of Dream to Eio using cohttp-eio. On HTTP/1, it works better than the previous port attempts (#254, #194) and should be more maintainable, including because cohttp-eio offers a direct-style interface (the advantages of which this port still needs to propagate further into the Dream code), but a lot of features will be missing even after the port is complete -- HTTP/2 support, GraphQL, and others, because the corresponding libraries are not ported to Eio or not available with Cohttp. A lot of features, like Caqti integration, that will be available when this port is done, are temporarily disabled for now, and will be added back as the port progresses. WebSockets will require additional work.

The current status of the port is that I am working through various deadlocks and corner cases of using Eio "promises" together with fibers, which are caused by the existing code of Dream assuming a sort of promise-callback soup, where any callback can be called on the one big stack under Lwt.main and make progress, while with fibers and Eio blocking the wrong fiber to wait on a promise can block a stack that would have been used to make progress to resolve that promise. The semantics are therefore substantially different. This was a major problem with the approach in #194 and #254. It is substantially negated by thoroughly using direct style wherever possible, rather than promises or callbacks.