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

Client: rs
diff --git a/lib/rs/Makefile.am b/lib/rs/Makefile.am
index dd1c03b..4abdff8 100644
--- a/lib/rs/Makefile.am
+++ b/lib/rs/Makefile.am
@@ -32,10 +32,12 @@
 
 check-local:
 	$(CARGO) fmt --all -- --check
+	$(CARGO) clippy --all -- -D warnings
 	$(CARGO) test
 
 all-local:
 	$(CARGO) fmt --all -- --check
+	$(CARGO) clippy --all -- -D warnings
 	$(CARGO) build
 
 clean-local:
diff --git a/lib/rs/test/Makefile.am b/lib/rs/test/Makefile.am
index 19056a6..017a2c4 100644
--- a/lib/rs/test/Makefile.am
+++ b/lib/rs/test/Makefile.am
@@ -29,6 +29,7 @@
 
 check: stubs
 	$(CARGO) fmt --all -- --check
+	$(CARGO) clippy --all -- -D warnings
 	$(CARGO) build
 	$(CARGO) test
 	[ -d bin ] || mkdir bin