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.
Technically, it should be a time_t.  time_t is mapped to a 32-bit int on 32-bit systems and a 64-bit int on 64-bit systems, for both Windows and Linux.  Fixed.