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.
I would add ,once to the date function - to improve the generated code, and be consistent with the behaviour of Today() - see GetDateYYYYMMDD in stringlib.cpp