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 #1776 (comment) 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 a88e110, we additionally:

• Remove IntoFuture from the trait bounds as it's no longer needed 60bcaed
• Remove try_poll as it's now redundant 75e53ff
• Remove IntoFuture entirely 4587897
• Remove sealing from TryFuture as any attempted implementation would be met with a conflicting implementation from the blanket implementation 9e15e25

[hlbarber]

A corresponding change can be made to TryStream.

[taiki-e]

See #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 #2508 (comment), #2344 will unblock this.