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

Client: rs
diff --git a/test/rs/Makefile.am b/test/rs/Makefile.am
index afb2cad..78db5ee 100644
--- a/test/rs/Makefile.am
+++ b/test/rs/Makefile.am
@@ -23,6 +23,7 @@
 precross: stubs
 	$(CARGO) build
 	$(CARGO) fmt --all -- --check
+	$(CARGO) clippy --all -- -D warnings
 	[ -d bin ] || mkdir bin
 	cp target/debug/test_server bin/test_server
 	cp target/debug/test_client bin/test_client