Solve InterviewCake's recursive string permutations problem

Write a function that returns the set of all of the possible permutations of an
input string. This function should be solved recursively.

scratch/deepmind/part_two/todo.org

@@ -28,7 +28,7 @@
 ** DONE MeshMessage
 ** DONE Find Repeat, Space Edition BEAST MODE
 * Dynamic programming and recursion
-** TODO Recursive String Permutations
+** DONE Recursive String Permutations
 ** TODO Compute nth Fibonacci Number
 ** TODO Making Change
 ** TODO The Cake Thief