Use IntoFuture for Mutex.lock

[LasseRosenow]

Rust 1.64 introduced the IntoFuture Trait. with this we could write:

let mutex = Mutex::new(0);
mutex.await;

instead of

let mutex = Mutex::new(0);
mutex.lock().await;

[Darksonn]

Ignoring the fact that this is incompatible with our minimum-supported-rust-version, that doesn't seem like a good idea. It seems less confusing to have an explicit lock method.

Also, I believe the .await of IntoFuture consumes the argument, which is not appropriate for locking a mutex.

[hawkw]

Additionally, this would be an API change that's somewhat inconsistent with std::sync::Mutex --- you don't lock a mutex by "calling" the mutex as though it were a function, you call its lock method. The use of IntoFuture here would be worth considering in a hypothetical world where std::sync::Mutex's API looked like this:

// if you could write this...
let mutex = std::sync::Mutex::new(0);
let _guard = mutex();

// ...then it might make more sense to write this
let mutex = tokio::sync::Mutex::new(0);
let _guard = mutex.await;

but since that's not the case, it's probably better to have a lock() method for consistency.