Usage of Router inside a DurableObject: need to move env, which is required to be a reference

Hi,

Apologies in advance if I'm missing something obvious here, I'm new to Rust. I'm trying to use `Router` in a `#[durable_object]`s `fetch` method like so:

```rust
use worker::*;

#[durable_object]
pub struct Chatroom {
messages: Vec,
state: State,
env: Env,
}

#[durable_object]
impl DurableObject for Chatroom {
fn new(state: State, env: Env) -> Self {
Self {
messages: vec![],
state,
env,
}
}

async fn fetch(&mut self, req: Request) -> worker::Result {
let router = Router::with_data(&self.messages);

router
.get("/", |_req, context| {
Response::ok(&format!("{} messages", context.data().len()))
})
.run(req, self.env)
.await
}
}
```

This fails with the error:

```
error[E0507]: cannot move out of `self.env` which is behind a mutable reference
--> src/chatroom.rs:27:23
|
27 | .run(req, self.env)
| ^^^^^^^^ move occurs because `self.env` has type `worker::Env`, which does not implement the `Copy` trait
```

This makes sense to me, as `Router#run` takes ownership of it's `env` argument, and since I'm taking a `&mut self` as a reference I can't move `self.env`. The `#[durable_object]` macro forces `fetch` to be `&mut self` so I can _only_ give references of `env`. I can't work out how you would use Router inside it?

Start with the DurableObject fetch entry point shown in src/chatroom.rs, then inspect Router::with_data and Router::run to understand their ownership requirements. Verify the behavior with the displayed Chatroom example; done means a Router can be used from the mutable fetch method without moving the stored Env.

Written by the indexing model from the issue text.