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 found a set of code that maps Posix time_t functions (mktime, localtime, and gmtime) to underlying Windows native time functions and back again.  This would give us the ability to use years in the 1601-3000 range.  64-bit Linux systems should work well beyond those ranges, as (I believe) all of our supported platforms support negative time_t values correctly.  Anyway, I'm in the process of validating those functions and will hopefully integrate them today.

We may want to consider putting some boundaries on the dates we support, either within the code or the documentation.