meetup: use calendar instead of core

[marionebl]

There has been previous work to remove the dependency on Core in favor of Base.

meetup was the last exercise depending on Core. Should this be merged we could remove Core and Core_kernel from the dependencies completely by migrating the test-generator to Base, too.

[sshine]

Very nice!

For the README update, this will probably break configlet, but let's fix that at a later time when we get configlet hooks into CI. The preferred way is to keep extra exercise-specific README information in a .meta/hints.md file that configlet copies into the README.