Switch to stable Futures

[tomaka]

Instead of using futures 0.1, let's use futures from the Rust standard library!

[tomaka]

Note that after async/await get stable, the general intention is that it should be possible to use non-'static futures.

In other words, we could write something like this:

impl Foo {
    // the returned `Future` borrows `self`
    fn next_item(&mut self) -> impl Future { ... }
}

This will considerably reduce the need for the Stream trait, which isn't stable, to point where we might be able to eliminate its use.

[tomaka]

Stables futures don't have any equivalent to loop_fn, which grandpa unfortunately uses.
We're probably going to have to wait for async/await.

Here's my WIP, for future reference: https://gist.github.com/tomaka/8dbe4b2c2ca69a34a72000deb3c5ad92

[bkchr]

Why not just implement a custom Future? Will make the code more readable.

[kigawas]

And consider using tokio 0.2 when it's ready

[expenses]

Still waiting on:

• hyper
• libp2p
• jsonrpc (or probably a switch to jsonrpsee)