await builders directly

[danieleades]

It would be nice to await builders directly.

There's a nice explanation of this pattern here - https://blog.yoshuawuyts.com/async-finalizers/

it will allow us to rewrite this-

let index = Index::init(root, download)
    .build()
    .await?;

like this-

let index = Index::init(root, download)
    .await?;

There are two options for doing this

1. implement Future for the builders. This isn't too hard, but it's not that clean, and makes the code look a little unapproachable.
2. implement IntoFuture for the builders. This relies on an unmerged nightly feature, and has been stuck in limbo for a while - Re-land "add IntoFuture trait and support for await" rust-lang/rust#68811