Futures-util dev dependency should require alloc feature

[Darksonn]

We had the following CI failure on Tokio's run-hyper-tests job:

error[E0425]: cannot find function `join_all` in module `future`
   --> tests/support/mod.rs:477:26
    |
477 |         Box::pin(future::join_all(client_futures).map(|_| ()))
    |                          ^^^^^^^^ not found in `future`
    |
note: found an item that was configured out
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-util-0.3.30/src/future/mod.rs:81:5
    |
81  | mod join_all;
    |     ^^^^^^^^
    = note: the item is gated behind the `alloc` feature
note: found an item that was configured out
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-util-0.3.30/src/future/mod.rs:83:26
    |
83  | pub use self::join_all::{join_all, JoinAll};
    |                          ^^^^^^^^
    = note: the item is gated behind the `alloc` feature

It seems like the futures-util crate has an alloc feature that should be enabled.

Most likely, it was previously enabled by some other dependency that no longer enables alloc.

[seanmonstar]

Interesting, none of the recent commits seem related. Probably a dependency no longer enables it.

[Darksonn]

Tokio's CI will currently use the newest commit that has a tag on it (as a proxy for latest release). Can we add a tag on this commit?

[seanmonstar]

Sure, I push a tag with a name discouraging explicitly relying on it.