Make odate work with OCaml >4.06 and OPAM >2.0

[dmbaturin]

Right now oDate cannot be built and installed with OCaml >4.06 and OPAM 2.x. This is unfortunate because there's no better library for parsing datetime strings!
calendar is fine for parsing dates, but its implementation doesn't work right with formats that include both date and time (ocaml-community/calendar#35).

Thus I really would love to see a new release of oDate. These patches update the code to use Stdlib rather than old-style Pervasives. Since there's only one dependent package (coclobas), I believe it will not cause big problems.
@smondet Please confirm or deny whether you are fine with this change.

The OPAM file now works with 2.0 and should be ready for a PR. I successfully did opam pin add . with OCaml 4.10 and opam 2.0.7:

utop # ODate.Unix.From.string (ODate.Unix.From.generate_parser "%F %H:%M" |> Option.get) "2020-11-17 06:05" |> ODate.Unix.Printer.to_iso ;;
- : string = "2020-11-17T06:05:00Z"

[smondet]

@dmbaturin LGTM 👍

[dmbaturin]

It's possible to make it compatible with older OCaml versions using stdlib-shims, but I believe it's better to keep it legacy-free, at least until users of old OCaml versions come to demand support. :)

[hhugo]

Testing is using should that is not compatible with ocaml > 4.06

[dmbaturin]

@hhugo I can help with rewriting tests using something that does work. We need to decide what to use though.

Do you consider non-working tests a blocker for the OPAM release?

[hhugo]

I'd be happy to switch to expect tests https://github.com/janestreet/ppx_expect

[hhugo]

#6

[hhugo]

I'll try to make a release today