move old files to a separate directory

old_curriculum/variables/variables4.rs

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+// variables4.rs
+// Make me compile! Scroll down for hints :)
+
+fn main() {
+    let x: i32;
+    println!("Number {}", x);
+}
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+// Oops! In this exercise, we have a variable binding that we've created on
+// line 5, and we're trying to use it on line 6, but we haven't given it a
+// value. We can't print out something that isn't there; try giving x a value!
+// This is an error that can cause bugs that's very easy to make in any
+// programming language -- thankfully the Rust compiler has caught this for us!