std/time: add the Weekday type alias and relevant constants

std/time/time.jule

@@ -31,6 +31,17 @@ const October = Month(10)
 const November = Month(11)
 const December = Month(12)
 
+// Specifies a day of the week (Sunday = 0, ...).
+type Weekday: int
+
+const Sunday = Weekday(0)
+const Monday = Weekday(1)
+const Tuesday = Weekday(2)
+const Wednesday = Weekday(3)
+const Thursday = Weekday(4)
+const Friday = Weekday(5)
+const Saturday = Weekday(6)
+
 // A Time represents an instant in time with nanosecond precision.
 //
 // Zero-value indicates the beginning of Unix time, i.e. zero seconds.
@@ -74,7 +85,7 @@ impl Time {
 	}
 
 	// Returns the day of the week specified by the time.
-	fn Weekday(self): int {
+	fn Weekday(self): Weekday {
 		ret absWeekday(self.abs())
 	}
 
@@ -120,10 +131,10 @@ fn absClock(abs: u64): (hour: int, minute: int, second: int) {
 	ret
 }
 
-fn absWeekday(abs: u64): int {
+fn absWeekday(abs: u64): Weekday {
 	// January 1 of the absolute year, like January 1 of 2001, was a Monday.
-	sec := (abs + 1*secPerDay) % secPerWeek
-	ret int(sec) / secPerDay
+	sec := (abs + u64(Monday)*secPerDay) % secPerWeek
+	ret Weekday(int(sec) / secPerDay)
 }
 
 fn absDate(abs: u64, full: bool): (year: int, month: Month, day: int, yday: int) {
@@ -225,7 +236,7 @@ fn Unix(mut sec: i64, mut nsec: i64): Time {
 // Absolute time.
 struct AbsTime {
 	Day:     int
-	Weekday: int
+	Weekday: Weekday
 	YearDay: int
 	Month:   Month
 	Year:    int