Free the allocated strings on the caller side in Raindrops exercise

https://github.com/exercism/odin/tree/main/exercises/practice/raindrops
https://exercism.org/tracks/odin/exercises/raindrops

Should we delete the strings we get from the `convert` function by ourselves? ATM the absence of deleting strings on the receiver side makes no sense and leads to inadequate solutions. (They use string builder and destroy it at the end of the `convert` function).