Support SQL dates in coq code

[joshuaauerbachwatson]

The SQLDate foreign type lacks an ocaml implementation, so our evaluators written coq generally choke on anything that contains the type. Ideally, there would be a true implementation in ocaml. For demo purposes, in the short run, it would perhaps suffice to write a dummy (error-throwing) implementation ocaml and replace the generated JS with callouts to our working SQLDate library in javascript.

[jeromesimeon]

Possible library for date/time in OCaml, but lacks a maintainer at the moment: https://github.com/ocaml-community/calendar