Add Pgx_value_ptime with ptime converters

[jsthomas]

This change addresses Issue #97 by introducing a new package called Pgx_value_ptime. This package is analogous to Pgx_value_core, but uses Ptime for date/date-time processing instead of Core.

The tests for Pgx_value_ptime are pretty much the same as those for Pgx_value_core, although I did add some extra tests for dates.

As I mentioned in the issue, I'm new to both this code base and OCaml in general, so please let me know if any of the code here isn't idiomatic and/or doesn't solve the problem the way you had in mind.

[jsthomas]

This is failing in Circle with the error:

[ERROR] No solution for pgx & pgx_async & pgx_lwt & pgx_lwt_mirage & pgx_lwt_unix & pgx_unix & pgx_value_core & pgx_value_ptime: The following dependencies couldn't be met:
          - pgx_value_ptime -> ptime >= v0.8.3
              no matching version

I'm not sure why this is, opam seems to show Ptime 0.8.5 as available.

[brendanlong]

I wonder if our build script is missing an opam update or something. I'll take a look.

[brendanlong]

The code looks great, thanks!

Let me try to figure out what's going on with CircleCI..

[brendanlong]

It might be worth pre-compiling the regexes as globals so we don't have to recompile them on each call. I'm not sure how slow that is in OCaml though so maybe it doesn't matter either way?

Suggested change

	let to_time' text =
	(*
	The time string can come in various forms depending on whether the
	Postgres timestamp used includes the time zone:
	Without timezone
	2016-06-07 15:37:46
	2016-06-07 15:37:46.962425
	With timezone
	2016-06-07 15:37:46-04
	2016-06-07 15:37:46.962425-04
	For the first one we need to indicate that it's a UTC time by appending
	a 'Z'. For the second one we need to append the minutes to the timezone.
	*)
	let open Re in
	let tz = seq [ alt [ char '-'; char '+' ]; digit; digit ] in
	let utctz = seq [ char 'Z'; eol ] |> compile in
	let localtz = seq [ tz; char ':'; digit; digit; eol ] |> compile in
	let localtz_no_min = seq [ tz; eol ] |> compile in
	let to_time' =
	(*
	The time string can come in various forms depending on whether the
	Postgres timestamp used includes the time zone:
	Without timezone
	2016-06-07 15:37:46
	2016-06-07 15:37:46.962425
	With timezone
	2016-06-07 15:37:46-04
	2016-06-07 15:37:46.962425-04
	For the first one we need to indicate that it's a UTC time by appending
	a 'Z'. For the second one we need to append the minutes to the timezone.
	*)
	let open Re in
	let tz = seq [ alt [ char '-'; char '+' ]; digit; digit ] in
	let utctz = seq [ char 'Z'; eol ] |> compile in
	let localtz = seq [ tz; char ':'; digit; digit; eol ] |> compile in
	let localtz_no_min = seq [ tz; eol ] |> compile in
	fun text ->

[brendanlong]

Looks like we needed 0.8.3 instead of v0.8.3. I wonder if the opam maintainers would be interested in adding a warning for when this happens.

[jsthomas]

Oops, thanks for figuring that out; I should've looked more closely and noticed the missing v.

[brendanlong]

No worries :)

[brendanlong]

Oh I just noticed your comment asked for feedback on the code. I think it looked pretty good to me. One part I'm wondering about is if we could avoid the regexes somehow by using functions built into ptime, but I took a quick look and didn't see any. We might want to consider some performance optimizations around it, like pre-compiling the regexes or trying the regexes in sequence instead of in parallel (so we won't need to run all of them in most cases), but we can do improvements to the internals like that in separate commits if/when the performance matters.

Overall, I thought the PR looked great.

[jsthomas]

Thanks for reviewing! This was a nice issue for learning more about how to use Ptime and about how postgres handles/presents date-times. The "good first issue" and "help wanted" labels were also helpful for finding something to work on.