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.
Good catch.  Since the incoming formats are well-defined but easily changed, and we don't constrain the formats at all, it's possible to get values from prior calls to simple_strptime() to bleed over to subsequent calls.  Fixed here as well as in slMatchDate().