rust: Panic dropping a future outside of an async context

Code such as this:

```rust
fn main() {
let (tx, rx) = wit_future::new(|| Ok(None));
drop(tx);
drop(rx);
}
```

will currently fail with:

```
thread 'main' (1) panicked at /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wit-bindgen-0.48.1/src/rt/async_support/waitable.rs:193:13:
assertion failed: !task.is_null()
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

(more context/discussion [are here](https://bytecodealliance.zulipchat.com/#narrow/channel/219900-wasi/topic/wasi-testsuite.20update.20for.20wasip3/near/562474600))

I'm not sure how best to handle this myself. It's not clear how frequently this will come up and if this means we basically need more libc integration or what.