Mirostat sampling crashes

[mdegans]

This is because of some API changes to the "Candidates" container in drama_llama. the container used to have a fixed size and the sampling functions would either return slices or Vecs. When the interface became more functional, taking self by value and returning self, it broke some of the sampling methods.

They're not covered, so it's unsurprising. To fix this we should generate some test data and cover the sampling functions so this can't happen again.