Fix clippy warnings (#40)

Add explicit `dyn`s

Remove let binding

Use +=

Return false

Derive Default for TcpConnector

Squash if/else

Remove unnecessary return keywords

Use is_empty()

Fix clippy attribute

Allow mut_from_ref

actix-rt/src/runtime.rs

@@ -40,7 +40,7 @@ impl Error for RunError {
     fn description(&self) -> &str {
         self.inner.description()
     }
-    fn cause(&self) -> Option<&Error> {
+    fn cause(&self) -> Option<&dyn Error> {
         self.inner.source()
     }
 }