prime-factors: Add solution template (#440)

* Add solution template for prime-factors

* Use the template function's parameter

This way, the learner doesn't hit `unused variable` warnings when they
first compile the example.

Author: nathanielknight

exercises/prime-factors/src/lib.rs

@@ -0,0 +1,3 @@
+pub fn factors(n: u32) -> Vec<u32> {
+    unimplemented!("This should calculate the prime factors of {}", n)
+}