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.
No, but it does go through different code paths.  In particular it highlighted a bug in the constant folding!  It could be that there is a bug in the way the functions would be called from generated c++, but that is a lot less likely.