Update ahash to 0.8.1

[emilk]

Previously, egui users needed to add this to their Cargo.toml:

[target.'cfg(target_arch = "wasm32")'.dependencies]
getrandom = { version = "0.2", features = ["js"] }

(see #2009).

Thanks to tkaitchuck/aHash#131 we can use ahash with a fixed seed, and let users opt-in to a random seed if they want it.