It segfaults under multicore: see savonet/ocaml-ssl#76

This is a proof-of-concept port of Dream to Eio. Most of the public API
in dream.mli has been changed to no longer use promises and the main
tutorial examples (`[1-9a-l]-*`) have been updated and are working.
The documentation mostly hasn't been updated.

Internally, it's still using Lwt in many places, using Lwt_eio to
convert between them.

The main changes are:

- User code doesn't need to use lwt (or lwt_ppx) now for Dream stuff.
  However, the SQL example still uses lwt for the Caqti callback.

- Dream servers must be wrapped in an `Eio_main.run`.
  Unlike Lwt, where you can somewhat get away with running other
  services with `Lwt.async` before `Dream.run` and relying on the
  mainloop picking them up later, everything in Eio must be started
  from inside the loop. Personally, I think this is clearer and less
  magical, making it obvious that Dream can run alongside other Eio
  code, but obviously Dream had previously made the choice to hide
  the `Lwt_main.run` by default.

- `Dream.run` now takes an `env` argument (from `Eio_main.run`),
  granting it access to the environment. At present, it uses this
  just to start `Lwt_eio`, but once fully converted it should also
  use it to listen on the network and read certificates, etc.

Error handling isn't quite right yet. Ideally, we'd create a new Eio
switch for each new connection, and that would get the errors. However,
connection creation is currently handled by Lwt. Also, it still tries to
attach the request ID to the Lwt thread for logging, which likely won't
work. I should provide a way to add log tags to fibres in Eio.

Note: `example/k-websocket` logs `Async exception: (Failure "cannot write to closed
writer")`. It does that on `master` with Lwt too.

After accepting a connection we convert it to a Lwt_unix.file_descr and
continue as before.

The `stop` argument has gone, as you can now just cancel the Eio fibre
instead. Note that this will cancel all running requests too (unlike the
previous behaviour, where it only stopped accepting new connections).

Segfaults due to savonet/ocaml-ssl#76

Just fixes some deprecation warnings.

Anton says he prefers not passing the clock as an argument.

upload.ml uses Lwt_streams, we convert them directly with lwt_eio

This reverts commit 6f608d0.

Using capabilities shouldn't be a big problem?
Eio.Path.with_open_dir seems to work quite nicely if needed

Random.initialize isn't called any more. I think its unnecessary as any
crypto calls will fail explicitly (due to performing an effect without a
handler). I did not remove it as I'm not that familiar with mirage and
didn't want to silently break crypto

It was picking up Mirage dependencies. The build target has been updated
in master so as to be useful in a Dune workspace and in this PR, without
picking up extra dependencies.