feat(iterators5): Introduce fibonacci exercise

Every good programming exercise book should include the fibonacci
numbers!

One possible solution:

    (0..i).fold((0, 1), |(a, b), _| (b, a + b)).1

Signed-off-by: Corin Lawson <corin@responsight.com>

exercises/standard_library_types/iterators5.rs

@@ -0,0 +1,61 @@
+// iterators5.rs
+
+pub fn fibonacci(i: u64) -> u64 {
+    // Complete this function to return the ith fibonacci number
+    // Do not use:
+    // - return
+    // For extra fun don't use:
+    // - imperative style loops (for, while)
+    // - let
+    // For the most fun don't use:
+    // - recursion
+    // Scroll down for hints.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn fibonacci_of_1() {
+        assert_eq!(1, fibonacci(1));
+    }
+    #[test]
+    fn fibonacci_of_2() {
+        assert_eq!(2, fibonacci(2));
+    }
+
+    #[test]
+    fn fibonacci_up_to_10() {
+        assert_eq!(
+            "[1, 1, 2, 3, 5, 8, 13, 21, 34, 55]",
+            format!("{:?}", (0..10).map(fibonacci).collect::<Vec<u64>>())
+        );
+    }
+}
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+// In an imperative language you might write a for loop to iterate through
+// add and keep track of the last two values into a mutable variables.
+// Or you might write code more functionally with recursion and a match clause.
+// But you can also use ranges and iterators to solve this in rust.

info.toml

@@ -217,3 +217,7 @@ mode = "test"
 [[exercises]]
 path = "exercises/standard_library_types/iterators4.rs"
 mode = "test"
+
+[[exercises]]
+path = "exercises/standard_library_types/iterators5.rs"
+mode = "test"