Easy way to update all toolchains

[danielparks]

There needs to be an easy way to upgrade all toolchains and rustup itself as well.

rustup { 'user':
  ensure     => latest,
  update     => all,
  toolchains => ['stable', 'nightly'],
}

What are other potential values for update? Should it be a value for ensure instead, e.g. latest_all?

Additionally, we may not want to run rustup update every puppet run. A cron job may be preferable.