Add time.weekday built-in function

[tsandall]

We currently have built-in functions to convert nanoseconds to year, month, day and hour, minute second (time.date and time.clock respectively.) In some cases, it makes sense to express policies in terms of days of the week. To support this, OPA should include a time.weekday built-in function that takes nanoseconds since epoch and returns the day of the week.

For example:

> time.weekday(time.now_ns())
"Monday"

Add built-in functions is relatively straightforward. Here's an example: db34907.

It should be possible to just convert nanoseconds into a time.Time struct in Go and then call time.Time#Weekday to get the day of the week.

This code can into topdown/time.go