Add Date and Time Pgx.Values for Lwt backend

[anuragsoni]

While prototyping a mirage backend for pgx (#67) I noticed that the Date/Time convertors are missing (they are present in the async backend).

For the lwt runtime maybe we can look at ptime (Calendar depends on unix so won't be usable on mirage) to provide similar support to create Pgx values?

In theory we could use the same functions as we do for the async runtime since they are using the date/time values from core_kernel which should be portable, but i'm not sure it'd be worth it to pull in core_kernel for the lwt runtime as i'm not sure if many people would use that combination.

[brendanlong]

Yeah, the reason we only expose it for Pgx_async is that we didn't want to push the full Core dependency on everyone (I'm not sure if Core_kernel even existed at the time).

I wonder if we could write the Pgx_value modules in a way where people can just depend on the ones they want, and then provide pgx_values_core.opam, pgx_values_ptime.opam, etc.

[brendanlong]

After #75, it's possible to use Pgx_lwt + Pgx_value_core. I think a Pgx_value_ptime would still be a good idea though.

[paurkedal]

Yes, ptime is a very minimal library and also quite popular, so I think that would be a good addition.

[brendanlong]

I'm going to close this as complete since we have Pgx_value_core now, and I opened #97 for ptime.