fix: optimize dayOfWeek & dateFromEpoch routines in date

[MaxGraey]

floorDiv(year, 400)  ->  floorDiv(floorDiv(year, 100), 4) 

Since arithmetic shift year >> 2 is equal to floorDiv(year, 4):

century = floorDiv(year, 100)
floorDiv(century, 4)  ->  century >> 2

And full simplifcation:

floorDiv(year, 4) - floorDiv(year, 100) + floorDiv(year, 400)

->

(year >> 2) - century + (century >> 2)

Also added couple of tests

• I've read the contributing guidelines
• I've added my name and email to the NOTICE file