Improve `Date` parser demo

[stephencelis]

Incorporates a helper from @juri's parser to improve things in our demo a bit.

Benchmarked against Parse3339 here and it's nice to see how well the relatively naive combinators performed by comparison:

name                      time         std        iterations
------------------------------------------------------------
Date.Parser                3583.000 ns ±  18.18 %     355076
Date.Parse3339             3083.000 ns ±  19.16 %     448056
Date.DateFormatter        21708.000 ns ±   7.24 %      63634
Date.ISO8601DateFormatter 26958.000 ns ±   5.17 %      51322

[stephencelis]

@BobbyRohweder I'm not sure how you're measuring it, but DateComponents.date is quite a bit slower:

Date.Parser (tm)                   3583.000 ns ±  18.18 %     355076
Date.Parser (DateComponents.date) 10750.000 ns ±  11.59 %     120687