write_fmt fails inside spawned task

It turns out that [`Arguments`](https://doc.rust-lang.org/std/fmt/struct.Arguments.html#impl-Send) is non-`Send`, which means passing it to a spawned Future may not work because the future itself is `Send`.

```rust
use async_std::task;
use async_std::prelude::*;
use async_std::io;

fn main() {
task::block_on(async {
task::spawn(async {
// This doesn't compile
io::stderr().write_fmt(format_args!("hello")).await?;
}).await
})
}
```

This is causing some bugs. Posting this for future reference.