rust: enable slice_flatten feature and provide it through an extension trait

rust: enable slice_flatten feature and provide it through an extension trait

In Rust 1.80, the previously unstable `slice::flatten` family of methods
have been stabilized and renamed to `slice::as_flattened`.

This creates an issue as we want to use `as_flattened`, but need to
support the MSRV (which at the moment is Rust 1.78) where it is named
`flatten`.

Solve this by enabling the `slice_flatten` feature, and providing an
`as_flattened` implementation through an extension trait for compiler
versions where it is not available.

The trait is then exported from the prelude, making the `as_flattened`
family of methods transparently available for all supported compiler
versions.

This extension trait can be removed once the MSRV passes 1.80.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/all/CANiq72kK4pG=O35NwxPNoTO17oRcg1yfGcvr3==Fi4edr+sfmw@mail.gmail.com/
Acked-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Message-ID: <20251110-gsp_boot-v9-8-8ae4058e3c0e@nvidia.com>
Message-ID: <20251104-b4-as-flattened-v3-1-6cb9c26b45cd@nvidia.com>