Add yojson dependency and update Makefile

Makefile

@@ -4,16 +4,7 @@ all:
 	dune build @runtest @all
 
 bench:
-	dune exec -- ./bench/bench_http.exe
-	dune exec -- ./bench/bench_condition.exe
-	dune exec -- ./bench/bench_buf_read.exe
-	dune exec -- ./bench/bench_mutex.exe
-	dune exec -- ./bench/bench_yield.exe
-	dune exec -- ./bench/bench_promise.exe
-	dune exec -- ./bench/bench_stream.exe
-	dune exec -- ./bench/bench_semaphore.exe
-	dune exec -- ./bench/bench_cancel.exe
-	if ocamlc -config | grep -q '^system: linux'; then dune exec -- ./lib_eio_linux/tests/bench_noop.exe; fi
+	dune exec -- ./bench/bench_main.exe
 
 test_posix:
 	EIO_BACKEND=posix dune runtest

eio.opam

@@ -24,6 +24,7 @@ depends: [
   "alcotest" {>= "1.4.0" & with-test}
   "dscheck" {>= "0.1.0" & with-test}
   "odoc" {with-doc}
+  "yojson" {>= "2.0.2" & with-test}
 ]
 conflicts: [
   "ocaml-base-compiler" {< "5.0.0~beta1"}