fix: return fn symbol from generated body

[rschmukler]

This commit changes the generate-defn return value to be the symbol of the newly generated function name. This is inline with the behavior of clojure.core/defn and clojure.core/defn-.

The reason this is useful is because some tools (namely cider / nrepl) rely on the return value of the defn for certain functionality, such as debugging and tracing. By returning the symbol, those tools go back to working with ghostwheel generated functions.