HPCC-8572 Std.Date functionality

Time support added to Std.Date; new timelib plugin; stringlib
modifications to support time; updated tests for new Std.Date
functionality; additional date manipulation functions for
Gregorian dates.
That was a bad comment change, but it happened to expose a bug.  The returned value was just struct tm.tm_yday which has the range 0-365.  To correctly match old behavior, the function now adds one to that value.  Also changed comment back to 1-366.  Also discovered that there were no Date.DayOfYear() tests; added a few.