makefile: add `cargo clippy` to tests if cargo is available

This linter mostly makes our code more idiomatic, less surprising, has good
suggestions and catches bugs. It's widely used in the Rust community and now
part of the default toolchain when using `rustup`.

Makefile

@@ -138,6 +138,7 b' tests:'
         # Run Rust tests if cargo is installed
 	if command -v $(CARGO) >/dev/null 2>&1; then \
 		$(MAKE) rust-tests; \
+		$(MAKE) cargo-clippy; \
 	fi
 	cd tests && $(PYTHON) run-tests.py $(TESTFLAGS)
 
@@ -155,6 +156,10 b' rust-tests:'
 	cd $(HGROOT)/rust \
 		&& $(CARGO) test --quiet --all --features "$(HG_RUST_FEATURES)"
 
+cargo-clippy:
+	cd $(HGROOT)/rust \
+		&& $(CARGO) clippy --all --features "$(HG_RUST_FEATURES)"
+
 check-code:
 	hg manifest | xargs python contrib/check-code.py