<chrono> Partially implement P0355R7

[SuperWig]

Description

Partially implemented #12;
[time.point.arithmetic] and [time.cal] (excluding I/O)

I've used a couple of Howard's algorithms, which I hope is okay as it's public domain

Consider these donated to the public domain

I originally used Howard's algorithm for year_­month_­day::operator sys_days() but I just saw LWG3206 in the issue thread and switched to it however it broke my test project's uses of constexpr due to the depth limit. I did eventually realise this was just a result of poor reading comprehension on my end.

Speaking of constexpr I had to disable the warning for negative integral => unsigned as uses of it in constexpr contexts fail to compile. No idea if that was the correct thing to do.

There is more than likely a better way of doing year_month_weekday::operator sys_days() const than how I implemented it. edit: I believe I've improved this.

During testing I found that this code triggers an ICE

constexpr auto test = 2019y / November / 24;
constexpr auto ymwdl =  2019y / November  / weekday_last{ Sunday };
constexpr sys_days _Last{ 2019y / November / last };

I think everything should be noexcept /* strengthened */? If so are they applied to declarations, definitions, or both?

Checklist

Be sure you've read README.md and understand the scope of this repo.

If you're unsure about a box, leave it unchecked. A maintainer will help you.

• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before automated testing is enabled on GitHub,
  leave this unchecked for initial submission).
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
• These changes were written from scratch using only this repository,
  the C++ Working Draft (including any cited standards), other WG21 papers
  (excluding reference implementations outside of proposed standard wording),
  and LWG issues as reference material. If they were derived from a project
  that's already listed in NOTICE.txt, that's fine, but please mention it.
  If they were derived from any other project (including Boost and libc++,
  which are not yet listed in NOTICE.txt), you must mention it here,
  so we can determine whether the license is compatible and what else needs
  to be done.

[SuperWig]

Maybe I should have squashed the commits 👀

Edit: decided to squash them as they added a lot of noise to the conversation. Surprised GitHub doesn't collapse them or something.

[StephanTLavavej]

Thanks for implementing a huge part of this huge C++20 feature, and for enduring over a year (!!!) of review! 😻 📅 📆 🚀 🎉

[SuperWig]

And thanks for enduring my sometimes questionable code 😅.

One thing though, is this one not an "issue"? #323 (comment)

[StephanTLavavej]

Thanks, missed that outstanding issue - basically, we're not super duper disciplined about our // SHOUTY COMMENT BANNERS, so I think this is fine. It can always be changed later, but I don't think it's a consistency problem as-is. (Basically I would accept a PR making it shoutier, but won't lose sleep if such a PR never appears.) 😹