Replace parts of futures-util with std APIs

[paolobarbolini]

Replaces the following futures-util APIs with std equivalents:

Before	After	Comment
futures_util::future::Future	std::future::Future	It was just a re-export
futures_util::future::poll_fn	std::future::poll_fn	Reimplements it in the same way
futures_util::future::ready	std::future::ready	Reimplements it in the same way
futures_util::ready!	std::task::ready!	Reimplements it in the same way
futures_util::pin_mut!	std::pin::pin!	Reimplements it in a similar way, but more powerful

std::pin::pin! should be the newest one, with a MSRV of v1.68.