Stats improvements

[Quafadas]

Here are a few improvements which would have a positive performance impact on the spearman calc (I believe)

• I realised that "sum" called down to an inefficient looking fold method, so I wrote that one by hand
• Element ranks now appends its results to a mutable array instead of constantly recreating immutable ones. I hope the impact here will be quite large
• spearman now takes a single pass through all it's arrays instead of calculating them seperately. - as hinted on discord.

I've left the other methods as is for the time being, as it sounds like you may wish to accomplish them other ways.

There's no pressure to merge!

[dragonfly-ai]

OK, good. This is a step in the right direction. Thanks!