Create iterators4.rs

Tackles https://github.com/carols10cents/rustlings/issues/15 . I think this allows for varying levels of difficulty depending on a users background. But ideally people can move there way up from 1, 2, 3 (Numbers below).

1. 
    let mut result = 1;
    for i in 1..x + 1 {
        result *= i;
    }
    result

2. 
    match x {
        0 | 1 => 1,
        x => x * factorial(x - 1),
    }
3. 
    (1..x + 1).product()

standard_library_types/iterators4.rs

@@ -0,0 +1,59 @@
+pub fn factorial(num: u64) -> u64 {
+    // Complete this function to return factorial of num
+    // Do not use:
+    // - return
+    // For extra fun don't use:
+    // - imperative style loops (for, while)
+    // - additional variables
+    // For the most fun don't use:
+    // - recursion
+    // Scroll down for hints.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn factorial_of_1() {
+        assert_eq!(1, factorial(1));
+    }
+    #[test]
+    fn factorial_of_2() {
+        assert_eq!(2, factorial(2));
+    }
+
+    #[test]
+    fn factorial_of_4() {
+        assert_eq!(24, factorial(4));
+    }
+}
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+// In an imperative language you might write a for loop to iterate through
+// multiply the values into a mutable variable. 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.