ISO8601 parsing test suite

[darrenldl]

Resources:

• https://github.com/micktwomey/pyiso8601
• https://github.com/ocaml-community/ISO8601.ml

[tmcgilchrist]

Adding a feature request here for a complementary function to

val of_iso8601 : string -> (t, string) Stdlib.result

val to_iso8601 : t -> string option
(** String conversion of ISO8601 format *)

[darrenldl]

I can make it an alias to to_rfc3339 unless it's meant to do more stuff not already done by to_rfc3339?

Which does remind me there's no ISO ordinal date printing...hm...