SwiftNeuralNetwork

This is roughly based on https://www.youtube.com/watch?v=w8yWXqWQYmU and other learning materials such as:

• 3Blue1Brown: https://www.youtube.com/watch?v=aircAruvnKk
• Sebastian Lague: https://www.youtube.com/watch?v=hfMk-kjRv4c

There are much better ways of doing what I'm doing here, like using Accelerate's BNNS APIs, or CoreML. This was very much a learning excercise for me.