'make bench' target that prints result to stdout

Unfortunately, it's hard to distinguish between a failed build or a
change to be promoted in the driver source.

I chose not to check for the driver exit status as it's likely to break
often.

Makefile

@@ -13,6 +13,15 @@ publish-docs:
 test :
 	dune runtest
 
+# Echo is disabled as the benchmarks results are printed on stdout.
+# '@bench' exit status is not checked as it's very likely to fail due to
+# needing promotion rather than because it really failed.
+.PHONY : bench
+bench:
+	-@dune build @bench
+	@dune promote driver-benchmarks.json
+	@cat driver-benchmarks.json
+
 .PHONY : coverage
 coverage :
 	dune build --instrument-with bisect_ppx @test/runtest --no-buffer -j 1 --force || true