Reformat rust code using 1.40 rustfmt and fail build if rustfmt fails (#2339)

diff --git a/tutorial/rs/Makefile.am b/tutorial/rs/Makefile.am
index 666331e..4aa05da 100644
--- a/tutorial/rs/Makefile.am
+++ b/tutorial/rs/Makefile.am
@@ -24,6 +24,7 @@
 
 all-local: gen-rs/tutorial.rs
 	$(CARGO) build
+	$(CARGO) fmt --all -- --check
 	[ -d bin ] || mkdir bin
 	cp target/debug/tutorial_server bin/tutorial_server
 	cp target/debug/tutorial_client bin/tutorial_client