* g-calend.adb (Has_53_Weeks): Rename to Last_Year_Has_53_Weeks. Add a
call to Jan_1_Day _Of_Week to optimize its performance.
(Is_Leap): Move the routine to the scope of Week_In_Year.
(Jan_1_Day_Of_Week): New routine in Week_In_Year which calculates the
weekday on which January 1 falls of Year - 1 and Year + 1. This function
avoids calling Time_Of and Split, thus making it more efficent.
(Week_In_Year): Reimplemented in oder to follow ISO 8601.
* g-calend.ads (Week_In_Year): Change comment to reflect new
implementation.