Move web server code into server module.

src/server.rs

@@ -0,0 +1,14 @@
+extern crate approx;
+
+pub async fn run_server() {
+    rocket::build()
+        .mount("/", routes![hello])
+        .launch()
+        .await
+        .unwrap();
+}
+
+#[get("/")]
+pub fn hello() -> &'static str {
+    "Hello, world!"
+}