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.
NOTE for information: Many of these tests will probably be done at compile time.  To ensure the functions are actually executed at runtime you need to add NOFOLD() around one of the parameters.  The other alternative is to use stored values as arguments.
Should be fine as it stands.