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.
It's not how we would do it for C++ - but it's probably more important that it works sensibly for ECL users.

I'm not sure if we support REAL8 as index fields, which would be a point in favour of using an integer representation,