Suggestion: Spawn tasks on first poll

*"A fundamental difference between Rust's futures and those from other languages is that Rust's futures do not do anything unless polled. "* [[0](https://github.com/rust-lang/rfcs/blob/master/text/2394-async_await.md#hot-async-functions)] I think following this behavior in this crate would make it more consistent with the rest of the ecosystem.

Current behavior: The snippet below keeps printing `"called"` indefinitely even though we did not await it.

```rust
crate::from_stream(async_std::stream::repeat(5usize))
.for_each(|_| async { println!("called") });
```