Refactor: split into multiple files

Given the size of the assignment, having everything in the same file was
OK. However, splitting into different files is a good option for better
maintainance once this API grows, and now is a good time to do it before
it gets more complicated.

project.clj

@@ -13,7 +13,7 @@
                  [metosin/muuntaja "0.6.11"]
                  [com.github.seancorfield/next.jdbc "1.3.1086"]
                  [org.xerial/sqlite-jdbc "3.51.1.0"]]
-  :main ^:skip-aot yohoho.core
+  :main ^:skip-aot yohoho.app
   :target-path "target/%s"
   :profiles {:uberjar {:aot :all
                        :jvm-opts ["-Dclojure.compiler.direct-linking=true"]}})