namespace: support :gen-class

Some examples here: https://clojuredocs.org/clojure.core/ns

And here: https://clojuredocs.org/clojure.core/gen-class

Source reference:

• https://github.com/clojure/clojure/blob/master/src/clj/clojure/genclass.clj
• https://github.com/clojure/clojure/blob/master/test/clojure/test_clojure/genclass.clj