Old slab version in dependencies breaks build

[K0bin]

async-executor has a dependency on a really old version of slab.

slab = "0.4.4"

The problem is that this version no longer compiles with any remotely recent version of Rust:

error[E0015]: cannot call non-const fn `Slab::<Waker>::new` in constant functions
   --> /home/robin/.cargo/registry/src/index.crates.io-6f17d22bba15001f/async-executor-1.13.1/src/lib.rs:693:32
    |
693 |             active: Mutex::new(Slab::new()),
    |                                ^^^^^^^^^^^
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

This was fixed back in 2022 and shipped with slab 0.4.7.