chore: move population structs to separate module

feat(lib): add two point and n point crossovers

refactor: abstract pairing to n-ary

refactor: pass rng as argument

Instead of having the Rng stored
inside the structs, pass it through
the functions. This means it's no longer
necessary to pass perturbations etc. as mutable.

chore: move env to codes/eoa_lib

feat: add crossover