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.
I think at a minimum we need to cope with dates back to 1900 (probably the extent of birth days), 1600 would be good, but I wouldn't lose too much sleep if we didn't support back to 1AD.

However..  we do need to be able to convert dates back to 1900 to and from strings etc.  that may preclude using many of the standard functions.

I think the previous implementation had seconds_t(1) as 1/1/1AD - I would need to check the source - i.e. apply a bias to the time_t values.

@dabayliss what do you think the minimum requirements are?