APIs missing / APIs that need to be re-evaluated

[matheus23]

We're currently exposing

• futures_util::TryFutureExt
• futures_util::TryStreamExt

And we might want to re-evaluate exposing them. I thought I added them for a good reason, but I can't grep any uses for them in iroh now.

We might also want to not expose futures_lite::io::AsyncRead and friends, given we usually use tokio's AsyncRead.

We've also found some APIs that might be nice-to-have, e.g.

• futures_util::FutureExt::flatten_stream, which doesn't exist in futures_lite::FutureExt, and we use in iroh.
• futures_util::FutureExt::map, which also doesn't exist in futures_lite::FutureExt, but is used in iroh-gossip.

We should also take another look at std::pin::pin! as a replacement for tokio::pin! or futures_lite::pin!. It doesn't seem to be a direct replacement, but we just might've been holding it wrong.