Add Date and Time Pgx.Values for Lwt backend

[anuragsoni]

Add Date and Time Pgx.Values for Lwt backend

[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.

[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.

[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.