Add IntoStream for all Iterator<_>

[matthiasbeyer]

Motivation

I just started recently using tokio (or writing async-await rust), and I found it very inconvenient (and slightly harder to read) to write

let i = collection.iter()
    .map(something);

tokio::stream::iter(i)
    .map(something_else)
    //...

But instead I wanted to write

collection.iter()
    .map(something)
    .into_stream()
    .map(something_else)
    // ...

Which is the problem I try to solve with this PR.

Solution

This PR thus implements an extension trait for all std::iter::Iterators which adds the into_stream() conveniance function.

Note

Please note that this is my first PR to this project and although I've read the contribution guidelines, I want to apologize in advance for any mistakes made.

Thank you for this awesome library!

[taiki-e]

Thanks for the PR. I think this PR's approach has some issues:

• APIs that allow all iterators to be used as streams unconditionally without additional annotations may not be preferable, as iterators may block threads.
• If you adopt the name IntoStream, for consistency with IntoIterator/IntoFuture, it preferable to be implemented in all streams, not all iterators. (And implement IntoStream for some iterators/collections)
• If implement IntoStream for all streams, since Stream is an upstream crate (futures-core)'s trait, adding IntoStream in a downstream crate (tokio) has some limitations. (see async-rs/async-std#213 and rust-lang/futures-rs#1879) (In this case, I recommend doing in upstream)

[Darksonn]

This should probably go in the futures crate, modulo @taiki-e's concerns.

[matthiasbeyer]

Okay, thanks for your time!