Faster implementation of getSolutions

[julian-zucker]

Describe the refactor you'd like.
StandardPopulation.getSolutions should be rewritten to be faster and more space-efficient.

Why should the refactor happen?
Right now, StandardPopulation.getSolutions is implemented by converting the population to a vector, shuffling the whole thing and taking the first n elements. This is easy code to write and to understand, but obviously slow on large populations. (Populations shouldn't be large, but if they are, it's unacceptable for Evvo to grind to a halt.)