Enable clippy in all Rust targets and ensure that all existing code is clippy-clean (#2341)

Client: rs
diff --git a/tutorial/rs/Makefile.am b/tutorial/rs/Makefile.am
index 4aa05da..13f6707 100644
--- a/tutorial/rs/Makefile.am
+++ b/tutorial/rs/Makefile.am
@@ -25,6 +25,7 @@
 all-local: gen-rs/tutorial.rs
 	$(CARGO) build
 	$(CARGO) fmt --all -- --check
+	$(CARGO) clippy --all -- -D warnings
 	[ -d bin ] || mkdir bin
 	cp target/debug/tutorial_server bin/tutorial_server
 	cp target/debug/tutorial_client bin/tutorial_client