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.
Completed the Windows-only code that correctly uses negative time_t values.  Now most Gregorian dates will work (most, because the Gregorian calendar starts around 1582 but Windows goes back to only 1601).  I added a IsValidGregorianDate() function to see if a Date_t value will work with all of these functions, and I've updated the documentation (citing the need for a Gregorian date) where necessary.