move old files to a separate directory

old_curriculum/variables/variables3.rs

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+// variables3.rs
+// Make me compile! Scroll down for hints :)
+
+fn main() {
+    let x = 3;
+    println!("Number {}", x);
+    x = 5;
+    println!("Number {}", x);
+}
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+// In Rust, variable bindings are immutable by default. But here we're trying
+// to reassign a different value to x! There's a keyword we can use to make
+// a variable binding mutable instead.