Add some exercises about functions!

functions/functions3.rs

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+// Make me compile! Scroll down for hints :)
+
+fn main() {
+    call_me();
+}
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+fn call_me(num: i32) {
+    for i in 0..num {
+        println!("Ring! Call number {}", i + 1);
+    }
+}
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+// This time, the function *declaration* is okay, but there's something wrong
+// with the place where we're calling the function.