WIP: Transition to eio (version with Eliom_stream)

[balat]

This branch is an experimental version of Eliom using Eio instead of Lwt.

To test it:

opam switch create ocsigen-eio 5.2.0
eval $(opam env --switch=ocsigen-eio)
opam pin -n add eio_js_backend https://github.com/ocaml-multicore/eio_js.git#js_events
opam pin -n add js_of_ocaml-eio https://github.com/ocaml-multicore/eio_js.git#js_events
opam pin -n add ocsipersist https://github.com/Julow/ocsipersist.git#to-eio
opam pin -n add ocsipersist-lib https://github.com/Julow/ocsipersist.git#to-eio
opam pin -n add ocsipersist-sqlite https://github.com/Julow/ocsipersist.git#to-eio
opam pin -n add ocsipersist-sqlite-config https://github.com/Julow/ocsipersist.git#to-eio
opam pin -n add ocsigenserver https://github.com/Julow/ocsigenserver.git#to-eio
opam pin -n add eliom https://github.com/ocsigen/eliom.git#to-eio2
opam pin -n add ocsigen-toolkit https://github.com/ocsigen/ocsigen-toolkit.git#to-eio
opam pin -n add ocsigen-start https://github.com/ocsigen/ocsigen-start.git#to-eio
opam install ocsigen-start
eliom-distillery -template os.pgocaml -name testocseio
cd testocseio
opam install .

Install postgresql, npm and sass. Then:

make db-init
make db-create
make db-schema
make test.byte

Then open http://localhost:8080