New Record Builder : <- syntax

[agu-z]

As discussed in this Zulip topic, we'd like to try out a new syntax for Record Builder fields that keeps the : after the label:

usersAndPosts = Task.succeed {
    users: <- Http.get "/users" |> Task.batch,
    posts: <- Http.get "/posts" |> Task.batch,
}

The intention is to make it clearer that we are assigning a record field, and obvious that this isn't introducing a new name to scope.