add binding to todo()

Author: cometkim

examples/run.res

@@ -16,6 +16,9 @@ let bench =
     b := a.contents - b.contents
     a := b.contents - a.contents
   })
+  ->Bench.todo("unimplemented bench")
 
 await bench.run(.)
+
 bench.table(.)->Console.table
+bench.todos->Array.map(({name}) => {name})->Console.table

src/Bench.res

@@ -43,6 +43,7 @@ type t = {
   reset: (. unit) => unit,
   table: (. unit) => Js.Dict.t<string>,
   tasks: array<Task.t>,
+  todos: array<Task.t>,
   results: array<option<Task.result>>,
 }
 
@@ -73,6 +74,9 @@ external add: (t, string, unit => unit) => t = "add"
 @send
 external addAsync: (t, string, unit => Js.Promise2.t<unit>) => t = "add"
 
+@send
+external todo: (t, string) => t = "todo"
+
 @send
 external remove: (t, string) => t = "remove"