Add clippy (#191)

* Organize imports

Use rustfmt to organize imports.  Resolve #162

* remove unused imports

* Fix clippy complaints

* add clippy github action

* remove comment for @arnaucube

src/backends/plonky2/mock/signedpod.rs

@@ -123,6 +123,9 @@ impl Pod for MockSignedPod {
     fn into_any(self: Box<Self>) -> Box<dyn Any> {
         self
     }
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
 
     fn serialized_proof(&self) -> String {
         self.signature.to_string()