Make build work (#546)

The mbedtls example has caused problem in the main build a number of
times. By making it a standalone `cargo new --bin`, we can keep it in
the source tree as a good example but avoid having it break the main
build.

Also, fix some clippy lints.

src/test/agent_test.rs

@@ -80,7 +80,7 @@ fn connection_reuse_with_408() {
     // pulls from the pool. If for some reason the timed-out
     // connection wasn't in the pool, we won't be testing what
     // we thought we were testing.
-    let resp = agent.post(&url).send_string("hello".into()).unwrap();
+    let resp = agent.post(&url).send_string("hello").unwrap();
     assert_eq!(resp.status(), 200);
 }
 

src/test/mod.rs

@@ -20,6 +20,7 @@ mod timeout;
 
 type RequestHandler = dyn Fn(&Unit) -> Result<Stream, Error> + Send + 'static;
 
+#[allow(clippy::type_complexity)]
 pub(crate) static TEST_HANDLERS: Lazy<Arc<Mutex<HashMap<String, Box<RequestHandler>>>>> =
     Lazy::new(|| Arc::new(Mutex::new(HashMap::new())));