Auto-generate and display test seeds to replay failures

[moodmosaic]

We're considering modifying the scenario! macro to generate a seed if none is given and print it. For example:

let seed = get_or_generate_seed();
println!("Seed: {}", seed);
println!("To reproduce: PROPTEST_RNG_SEED={} cargo test -- test_name", seed);

This would make each test run print its seed and replay instructions by default.