Drop DateTime dependency

Adds a module in t/lib that implements object stringification for tests requiring DateTime, thus allowing us to drop the dependency on DateTime which installs no less than 11 extra modules: DateTime, Test::Warnings, Params::Validate, DateTime::TimeZone, DateTime::Locale, Module::Implementation, Class::Singleton, List::AllUtils, Class::Load, Package::Stash, Package::Stash::XS.