Allow creating a CrossDate with a defined TimeZone.

[megri]

There's currently a bug in how Perfolation outputs zone offset strings (see discussion linked Scribe-issue below). To correct the issue and get reproducible tests we need a way to create a CrossDate that is timezone-independent; basically we want the timezone and corresponding zone-offset to be predictable.

For posterity, the discussion started at the end of this issue in Scribe:
outr/scribe#573

A brief discussion on discord with @darkfrog26 led us to try and add scala-java-time as a dependency, but it is currently not released for Scala Native 0.5+. The corresponding issue is tracked here:

cquiroz/scala-java-time#504