Replace unsafe `std::pin::Pin::new_unchecked` with `std::pin::pin!`

[tokarevart]

Currently to locally pin a future unsafe std::pin::Pin::new_unchecked is being used. This PR replaces that unsafe block with std::pin::pin! macro introduced in Rust 1.68 release.

[zesterer]

Thanks! I think I'm going to avoid merging this for now in order to avoid breaking the current MSRV (I should probably come up with a hard policy on this), but perhaps in a few months when the impact is less drastic I'll merge this.