Add async_try_stream to support ? operator in async stream

[taiki-e]

? operator can be used with the #[async_try_stream] attribute and async_try_stream_block! macro.

#![feature(generators)]
use futures::stream::Stream;
use futures_async_stream::try_async_stream;

#[async_try_stream(ok = i32, error = Box<dyn std::error::Error + Send + Sync + 'static>)]
async fn foo(stream: impl Stream<Item = String>) {
    #[for_await]
    for x in stream {
        yield x.parse()?;
    }
}

Closes #2

[taiki-e]

bors r+

[bors]

Build succeeded

• taiki-e.futures-async-stream