It's more an experiment than a real microframework, it's absolutely not recommended for production.
The contributing page should sound like a challenge :)
https://github.com/EmanueleMinotto/crystal/wiki/Contributing
They're not strange/crazy enough to make it a fun programming challenge, and for something that's intended for use (even non-production use) they make the code less usable.
"No namespaces" and "no new functions or classes" in particular don't make any sense. Anonymous functions are just like functions, except they're harder to identify in a stack trace.