Breakup routing module (#424)

The routing module had gotten quite hard to navigate so this breaks it
into smaller modules. No user facing changes.

src/routing/nested.rs

@@ -0,0 +1,69 @@
+use crate::body::BoxBody;
+use http::{Request, Response, Uri};
+use std::{
+    convert::Infallible,
+    task::{Context, Poll},
+};
+use tower::util::Oneshot;
+use tower::ServiceExt;
+use tower_service::Service;
+
+/// A [`Service`] that has been nested inside a router at some path.
+///
+/// Created with [`Router::nest`].
+#[derive(Debug, Clone)]
+pub(super) struct Nested<S> {
+    pub(super) svc: S,
+}
+
+impl<B, S> Service<Request<B>> for Nested<S>
+where
+    S: Service<Request<B>, Response = Response<BoxBody>, Error = Infallible> + Clone,
+    B: Send + Sync + 'static,
+{
+    type Response = Response<BoxBody>;
+    type Error = Infallible;
+    type Future = Oneshot<S, Request<B>>;
+
+    #[inline]
+    fn poll_ready(&mut self, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn call(&mut self, mut req: Request<B>) -> Self::Future {
+        // strip the prefix from the URI just before calling the inner service
+        // such that any surrounding middleware still see the full path
+        if let Some(tail) = req.extensions_mut().remove::<NestMatchTail>() {
+            UriStack::push(&mut req);
+            let new_uri = super::with_path(req.uri(), &tail.0);
+            *req.uri_mut() = new_uri;
+        }
+
+        self.svc.clone().oneshot(req)
+    }
+}
+
+pub(crate) struct UriStack(Vec<Uri>);
+
+impl UriStack {
+    fn push<B>(req: &mut Request<B>) {
+        let uri = req.uri().clone();
+
+        if let Some(stack) = req.extensions_mut().get_mut::<Self>() {
+            stack.0.push(uri);
+        } else {
+            req.extensions_mut().insert(Self(vec![uri]));
+        }
+    }
+}
+
+#[derive(Clone)]
+pub(super) struct NestMatchTail(pub(super) String);
+
+#[test]
+fn traits() {
+    use crate::tests::*;
+
+    assert_send::<Nested<()>>();
+    assert_sync::<Nested<()>>();
+}