Support fixed seed

[emilk]

Closes #126

This allows us to compile without either runtime-rng nor compile-time-rng (--no-default-features --features std).

Compiling without these flags will make ahash non-DOS-resistant.

This is meant for libraries (like egui) that

• Don't need DOS-resistance
• Don't want the added dependencies and compilation times of compile-time-rng
• Don't want to force either runtime-rng or compile-time-rng onto the library users
• Don't want to give users confusing error messages about missing getrandom when compiling for wasm32

[emilk]

❤️