Use a unique ID instead of `NeuronLocation`

[HyperCodec]

With the current NeuronLocation, it points to a specific index in one of the layers. This is problematic for deletions, since all of the indexes need to be recalculated every time, which is very expensive with multiple successive deletions.

[HyperCodec]

Since input and output never change, we could just have an enum with an index for input and output layers and an ID for the hidden layers.

[HyperCodec]

Hmm the problem with using unique IDs in some sort of map data structure is it will cause a lot of issues with crossover. Will defer this to after 1.0.0.