Support shared futures on no_std

[adavis628]

Currently, Shared futures are only available when the std feature is enabled because they use std::sync::Mutex internally. This PR changes this so that Shared futures will fall back to using a spinlock when std is not enabled.

Alternatively, Shared could be changed to be generic over a mutex trait (e.g. lock_api::RawMutex, though it isn't implemented for std::sync::Mutex) to allow arbitrary user implementations.

[taiki-e]

Thanks for the PR. I would prefer not to use spinlock by default around operations that would involve allocations even if it is no_std-only.

That said, it might be fine if this is an optional feature and spinlock is used only when std is disabled and the feature is explicitly enabled.

[adavis628]

I moved the spinlock behind an optional spin feature that will use a spinlock when std is disabled.

[taiki-e]

This needs to be #[cfg(any(feature = "std", all(feature = "alloc", feature = "spin")))]

https://github.com/rust-lang/futures-rs/actions/runs/12735209519/job/35493701034?pr=2868#step:5:199

running `cargo check --no-default-features --features unstable,spin` on futures (24/193)
      Checking futures-util v0.4.0-alpha.0 (/home/runner/work/futures-rs/futures-rs/futures-util)
  error[E0433]: failed to resolve: use of undeclared crate or module `alloc`
   --> futures-util/src/future/future/shared.rs:2:5
    |
  2 | use alloc::sync::{Arc, Weak};
    |     ^^^^^ use of undeclared crate or module `alloc`
    |
    = help: add `extern crate alloc` to use the `alloc` crate
  
  error[E0432]: unresolved imports `crate::task::waker_ref`, `crate::task::ArcWake`
    --> futures-util/src/future/future/shared.rs:1:19
     |
  1  | use crate::task::{waker_ref, ArcWake};
     |                   ^^^^^^^^^  ^^^^^^^ no `ArcWake` in `task`
     |                   |
     |                   no `waker_ref` in `task`
     |
  note: found an item that was configured out
    --> futures-util/src/task/mod.rs:31:24
     |
  31 | pub use futures_task::{waker_ref, WakerRef};
     |                        ^^^^^^^^^
  note: the item is gated behind the `alloc` feature
    --> futures-util/src/task/mod.rs:30:7
     |
  30 | #[cfg(feature = "alloc")]
     |       ^^^^^^^^^^^^^^^^^
  note: found an item that was configured out
    --> futures-util/src/task/mod.rs:23:23
     |
  23 | pub use futures_task::ArcWake;
     |                       ^^^^^^^
  note: the item is gated behind the `alloc` feature
    --> futures-util/src/task/mod.rs:22:7
     |
  22 | #[cfg(feature = "alloc")]
     |       ^^^^^^^^^^^^^^^^^
  
  error[E0432]: unresolved import `slab`
    --> futures-util/src/future/future/shared.rs:12:5
     |
  12 | use slab::Slab;
     |     ^^^^ use of undeclared crate or module `slab`