Simplified a lot of code which was unnecessarily generic.
Now using monads to manage the state of the random number generator
rather than passing it around by hand.
Also made some performance improvements, then increased the population
size so more combinations are tried in a similar length of time.
Stop when there has been little improvement across the last 5
generations, rather than the last 1. This generally allows a higher
scoring solution to be reached.
This allows us to reach an even better colour scheme rather than
stopping after a fixed number of generations.
For images which create a good colour scheme quickly, we avoid doing
unnecessary passes which don't lead to much improvement.
