[Feature Request] Make output the same for same RNG seed

[dandavison]

Ideally this would return the same value, but it doesn't currently (even after #57):

md5sum <(cargo run -- generate --explicit-seed=12345 2>/dev/null) <(cargo run -- generate --explicit-seed=12345 2>/dev/null)

I believe that this is due to the fact that serializing the WorkflowState HashMap yields keys and values in non-deterministic order. If so, it doesn't impact correctness, but it would still be reassuring to see the same output.

Fix this and add a test for same-seed-same-output.