Duration: support negative durations by prefixing a '-' before the P in ISO format

[apoelstra]

According to this MDN document there is an ECMAScript extension to ISO 8601 to allow signed durations by putting a + or - before the ISO duration format.

Internally we support signed durations -- we will parse "-60w" or "-60min", which we store in a time_t (whose signedness is technically implementation defined, but which is signed on all major compilers). However, when formatting these as ISO 8601 we get get garbed output like PT-1H-56M-9S, which we cannot parse and probably neither can anything else. (Taskwarrior, when asked to assign a negative duration to a duration-typed UDA, will store this garbled output but then reproduce it as PT0S, an unfortunate user experience.)

This PR updates Duration::formatISO to instead prepend a '-' before negative durations, and updates Duration::parse_designated to parse such things.

Alternative to #110, which simply blesses the garbled format by extending the parser to support it.

[lauft]

@apoelstra would be great if you could also provide test coverage for this. Otherwise, given that this has been lying around for some time (sorry about that), I will take over from here and provide it myself.

[apoelstra]

For sure! I am a bit busy for the next week or two -- feel free to add test coverage if you've got a moment. Otherwise I will do it, I just may be slow.

No worries about not getting to this for a little while. It's a busy life being an open-source maintainer.

[lauft]

@apoelstra I added test coverage (and adapted the code style a bit) and while at it encountered the following "problem":

For e.g. -P1Y1M1DT1H1M1S, the Duration::format() prints -396d 1:01:01 with a single leading minus sign.
I now wonder how such a "composite" duration would be read/interpreted by the user.

I would say for ISO (e.g. -P1Y1M1DT1H1M1S) or a single component duration (e.g. -1:01:01) the reading of the minus sign simple and obvious. I would interpret the latter rather as -3661s than -3539s (-1h + 1min + 1 s).

But how would you interpret something like -396d 1:01:01? Do you apply the minus to the whole expression, or just the days? Should it be rather -(396d 1:01:01) or -396d -1:01:01 then? 🤔

In general I would say a duration is always an positive value. The minus sign simply indicates direction or that it is "missing".

I am currently investigating the impact on this on Timewarrior, maybe you could have a look at Taskwarrior and give me your impression.