Add seedable generate functions

[Nystik-gh]

For a project I need to be able to reproduce the same generation multiple times so I've implemented a way to seed the random number generator, or provide your own generator.

If there is interest in merging such functionality.

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Add rand::Rng argument to States trait next() function.

Rename generate to generate_base which now takes an argument that implements rand::Rng
Rename generate_from_token to generate_from_token_base which now takes an argument that implements rand::Rng

Add new generate and generate_from_token functions that call the base functions with thread_rng().

Add the following seedable generate functions

• generate_with_seed(seed: u64)
• generate_with_rng<R: rand::Rng>(rng: R)
• generate_from_token_with_seed(token: T, seed: u64)
• generate_from_token_with_rng<R: rand::Rng>(token: T, rng: R)

Implement linked-hash-map for predictable hashmap insertion.

Add "seedable" feature

[RicardRC]

I'd like this to be merged, as I'm also interested in this feature. How could I contribute to this happening?

[aatxe]

The change looks fine to me, but we need to move the project over to GitHub Actions since it's still waiting for TravisCI which was killed long ago.