Calendar: Weeks spanning years

Unfortunately it breaks a lot of other things. I have a more thorough fix in the making, so just take it easy.. ;)

While I was working on this I reevaluated some of the test cases I checked in. They tried to let a week "belong" to only one year, so e.g.

Calendar.ISO.Year (Calendar.ISO.Week (2008, 1))

would return only year 2008 and not a range 2007..2008. That turned out to lead to many strange inconsistencies, so I've ditched that idea.

Fixing this to behave consistently introduces some compat issues though, so I need to get something working in the compat tree. Unfortunately it's not anywhere near as simple as just copying the whole thing, like Nilsson suggested. :P