create US holiday module

[mndrix]

Create a module library(julian/us/holidays) which implements forms like july_fourth, martin_luther_king_day, labor_day, etc. This is mostly a real world use case test to make sure Julian can:

• support real world date rules
• be extended by packs distributed independently

I suspect this module will also be fairly helpful for other users.

Logtalk has a date contribution which supports Easter Sunday calculations. Use that as a reference for how the calculations might be done.