Take advantage of 2018 edition.

- Remove explicit 'extern crate' fields where useful
- Fix mutability of variable (unrelated?)
- Add rbxmx dependency, which needs 2018 edition

server/src/web.rs

@@ -4,7 +4,12 @@ use std::{
     sync::{mpsc, Arc},
 };
 
-use rouille::{self, Request, Response};
+use rouille::{
+    self,
+    router,
+    Request,
+    Response,
+};
 use rbx_tree::{RbxId, RootedRbxInstance};
 
 use crate::{