Add sample files and snapshot generator

tests/snapshots/sample.modified.rs

@@ -0,0 +1,21 @@
+struct Rectangle {
+    width: u32,
+    height: u32,
+}
+
+fn main() {
+    let rect1 = Rectangle { width: 30, height: 50 };
+
+    println!(
+        "The perimeter of the rectangle is {} pixels.",
+        perimeter(&rect1)
+    );
+}
+
+fn area(rectangle: &Rectangle) -> u32 {
+    rectangle.width * rectangle.height
+}
+
+fn perimeter(rectangle: &Rectangle) -> u32 {
+    (rectangle.width + rectangle.height) * 2
+}