add weeks-in-year, extract leap-year-p, add two convenience functions

[vindarel]

Hi,

I can send different PRs if there's something to discuss.

I find tomorrow and yesterday very convenient.

I checked weeks-in-year manually.

Best,

[dlowe-net]

Sorry, I'm not interested in adding yesterday and tomorrow to the interface. I could be convinced to add weeks-in-years if there was a solid reason for it.