feat: replace futures crate with futures-util

[AsakuraMizu]

No description provided.

[George-Miao]

Hm, what's the difference?

[AsakuraMizu]

Hm, what's the difference?

Less dependencies, I think?

[Berrysoft]

futures-util is smaller, I think.

[Copilot]

Pull request overview

This PR replaces the futures crate dependency with the more lightweight futures-util crate, which provides only the utility types and traits without the full executor and macro machinery.

Key changes:

• Replaced futures dependency with futures-util in Cargo.toml
• Updated all import paths from futures:: to futures_util:: in source files
• Updated documentation references to reflect the new crate name
• Modified doc examples in mutex module to use project-specific imports

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Cargo.toml	Replaced futures with futures-util dependency and updated feature flags
src/waker_slot.rs	Updated import paths and documentation to reference futures_util::task::AtomicWaker
src/mutex/mod.rs	Updated import paths to futures_util::future::FusedFuture and modified doc examples to use synchrony:: instead of futures::lock

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[George-Miao]

Actually, I think we can remove future-util dependency for mutex. We are only using FusedFuture, which is not necessary.

[AsakuraMizu]

We are only using FusedFuture, which is not necessary.

Do you suggest that they should move to futures-core?

By the way, there's something very strange: AtomicWaker is actually defined in futures-core, but it's been #[doc(hidden)] and then re-exported in futures-util.

[George-Miao]

LGTM

[George-Miao]

@AsakuraMizu No, I mean we can completely get rid of futures in case of mutex

[George-Miao]

LGTM