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.
This will cause pain.  It appears that mktime() also does not handle negative time_t values.  I checked the Boost code for inspiration and even they throw an error on Windows when faced with negative (prior to 1970-01-01) times.

Windows supports dates and times back to 1601-01-01 via some other calls, so I will investigate those.  That doesn't help with even older dates, though.  Do we have to worry about them?