Update the neural network that learns XOR to the current horde-ad API

[Mikolaj]

Here's the old version:

https://github.com/Mikolaj/horde-ad/blob/15e83c6e6270cb844226909f89cb33d3f81cc9ef/example/XorNN.hs

I expect this to be a very interactive task, where I would provide a lot of hints (and GHC and test results would, too).