Small readability and idiom-related refactorings

- Use (:keyword (function-returning-a-map)) over ((function-returning-a-map) :keyword)
- Use (not= ...) over (not (= ...))
- Use `if` instead of `cond` when there are only two conditions
- Use `when` where the `else` case returns `nil`
- Use different signature for `repeatedly` to avoid an extra `take`
- Minor whitespace and indentation changes

I know this is nitpicking, but I couldn't help myself. :hamburger: