Add coding exercises for Facebook interviews

Add attempts at solving coding problems to Briefcase.

scratch/facebook/recursive-string-permutations.py

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+# permutations: no repeat characters
+
+def char_and_rest(i, xs):
+    return xs[i], xs[0:i] + xs[i + 1:]
+
+def permutations(xs):
+    if len(xs) == 1:
+        return [xs]
+    result = []
+    for c, rest in [char_and_rest(i, xs) for i in range(len(xs))]:
+        result += [c + perm for perm in permutations(rest)]
+    return result
+
+expected = ["cat", "cta", "act", "atc", "tca", "tac"]
+result = permutations("cat")
+print(result, expected)
+assert len(result) == len(expected)
+assert result == expected
+print("Success!")