Add some exercises about functions!

functions/functions1.rs

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+// Make me compile! Scroll down for hints :)
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+fn main() {
+    call_me();
+}
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+// This main function is calling a function that it expects to exist, but the
+// function doesn't exist. It expects this function to have the name `call_me`.
+// It expects this function to not take any arguments and not return a value.
+// Sounds a lot like `main`, doesn't it?