Constrain `Future::Output` in supertrait of `TryFuture`

[hlbarber]

The obstruction to

trait TryFuture: Future<Output = Result<Self::Ok, Self::Error>> {
    type Ok;
    type Error;
}

originally stated in https://github.com/rust-lang/futures-rs/issues/1776#issuecomment-518307092 no longer exists.

This PR provides an insight into what taking such a change now would look like now.

In addition to the minimal change to the TryFuture trait seen in a88e11011e33df8e6345b1a8527447f3dd5399c5, we additionally:

• Remove IntoFuture from the trait bounds as it's no longer needed 60bcaed15e76c7a1b585cb4b39ae0a5fc671a6e7
• Remove try_poll as it's now redundant 75e53ff6a9891471714330207fde9c02986f6cba
• Remove IntoFuture entirely 4587897773ed4792eaaa656ceb3589ebe2667f14
• Remove sealing from TryFuture as any attempted implementation would be met with a conflicting implementation from the blanket implementation 9e15e25d1cab668a0ae03cdd92fccfee90d9b2ad

[hlbarber]

A corresponding change can be made to TryStream.

[taiki-e]

See https://github.com/rust-lang/futures-rs/issues/2508 for the previous discussion.

[hlbarber]

This would require an MSRV bump to rustc version 1.52.

Failing 1.51 Passing 1.52

[taiki-e]

As mentioned in https://github.com/rust-lang/futures-rs/issues/2508#issuecomment-942407374, https://github.com/rust-lang/futures-rs/pull/2344 will unblock this.