Switch to IntoFuture trait instead of modifying SendFuture

This could clean up the code a bit, and IntoFuture is now [stable since 1.64](https://blog.rust-lang.org/2022/09/22/Rust-1.64.0.html#enhancing-await-with-intofuture). This seems to fit the usecase perfectly and could potentially help with a clean solution to #144.