Add support for ISO calendar week to Time

[straight-shoota]

This PR adds two methods to Time for handling ISO calendar week:

• Time#calendar_week returns the year and week number. The year is important because calendar weeks don't match up with calendar years and the calendar week might actually be in a different year than the regular calendar year (returned by #year).
• Time.week_date creates a Time instance from a week date specified by year, week number and day of week.

It also adds directives %G, %g, %V which utilize these methods to Time::Format.

The algorithms for calculating the week number and back are completely annotated in code. They're based on the description on Wikipedia and the implementation in Go's stdlib but with a few improvements.

Closes #2533
Depends on #6555

[jhass]

I didn't review the algorithm much, but this seems more than solid enough to be accepted for a first implementation. We can always sort out edge cases as they're discovered, if any.

[RX14]

needs a rebase after #6555.

[straight-shoota]

Rebased.

[bcardiff]

@straight-shoota Sorry, could you rebase again this on master so we can merge in a more tidy fashion?

[straight-shoota]

Sure, done. But I'm not sure what it adds, it could just be fast forwarded.

[bcardiff]

Thanks! A rebase will not leave a text reference to the PR. Squash or merge commit will. But is better to avoid long standing branch merged and keep the repo as thin as possible.