style(lib): address clippy code style warnings

* Remove unnecessary return statements.

* Combine identical `match` arms.

* Collapse nested conditional.

* Use `assert_ne` where applicable.

* Lifetime elision.

src/client/mod.rs

@@ -76,7 +76,7 @@ impl Client<HttpConnector, http::Body> {
 impl<C, B> Client<C, B> {
     /// Return a reference to a handle to the event loop this Client is associated with.
     #[inline]
-    pub fn handle<'a>(&'a self) -> &'a Handle {
+    pub fn handle(&self) -> &Handle {
         &self.handle
     }