Add some exercises about variables!

variables/variables1.rs

@@ -0,0 +1,6 @@
+// Make me compile!
+
+fn main() {
+    x = 5;
+    println!("x has the value {}", x);
+}

variables/variables2.rs

@@ -0,0 +1,10 @@
+// Make me compile!
+
+fn main() {
+    let x;
+    if x == 10 {
+        println!("Ten!");
+    } else {
+        println!("Not ten!");
+    }
+}

variables/variables3.rs

@@ -0,0 +1,8 @@
+// Make me compile!
+
+fn main() {
+    let x = 3;
+    println!("Number {}", x);
+    x = 5;
+    println!("Number {}", x);
+}

variables/variables4.rs

@@ -0,0 +1,6 @@
+// Make me compile!
+
+fn main() {
+    let x: i32;
+    println!("Number {}", x);
+}