Add cacheable library target

src/lib.rs

@@ -0,0 +1,3 @@
+pub fn message() -> &'static str {
+    "Hello, world! (sccache lib verification)"
+}

src/main.rs

@@ -1,3 +1,3 @@
 fn main() {
-    println!("Hello, world! (cache test v3)");
+    println!("{}", rust_hello_world::message());
 }