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.
Never mind.  I was looking at the wrong line in github.

I'm embarrassed that I got a memset() call wrong.  What a newbie mistake!

Anyway:  Fixed.