tokio::task::block_in_place can only be called inside spawned tasks

The following panics

```rust
#[test]
fn block_on_from_block_on() {
    let mut rt = runtime::Runtime::new().unwrap();
    rt.block_on_std(async {
        tokio_02::task::block_in_place(move || ());
    });
}
```

Easy workaround by wrapping the b`block_in_place` call with a `tokio::spawn`, but it had me confused for a bit.