`Observer` API

[HyperCodec]

Effectively just immutable access to the results of fitness. Will run before Eliminators and can be blanket implemented as a wrapper.

[HyperCodec]

They actually can't run before Eliminators because the Eliminator is the one that tests the genomes before eliminating. A much more complex implementation is required in order to continue, which I feel is a bit too much for 1.0.0. Will probably add in a later update though.

[HyperCodec]

I'm thinking that for Observers, we can just create an Observer trait with an observe method that takes in the genome and fitness and has no returns. Then, we can just create a struct that takes in an Observer and a FitnessFn and implements FitnessFn.

[HyperCodec]

added in cc0ebc4 (though I might want to polish later)