Add some exercises about functions!

functions/functions2.rs

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+// Make me compile! Scroll down for hints :)
+
+fn main() {
+    call_me(3);
+}
+
+fn call_me(num) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}
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+// Rust requires that all parts of a function's signature have type annotations,
+// but `call_me` is missing the type annotation of `num`.