Add assignment 2 skeleton

src/assignments/assignment02.rs

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+//! Assignment 2: Mastering common programming concepts (1/2).
+
+// TODO: use exercises in <https://doc.rust-lang.org/book/ch03-05-control-flow.html>

src/assignments/assignment02_grade.rs

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+#[cfg(test)]
+mod test {
+    use super::super::assignment02::*;
+}