Add MNIST inference example with CNN

Add MNIST inference example with CNN

Add one more implementation for MNIST which uses Conv2D layers, ref:
https://keras.io/examples/vision/mnist_convnet/. It achieves ~99%
accuracy on the MNIST test set and also performs better for user inputs.
This implementation expects a model in GGUF format. You can get one with
the 'mnist-cnn.py' script. Example usage:

$ ./mnist-cnn.py train mnist-cnn-model
...
Keras model saved to 'mnist-cnn-model'

$ ./mnist-cnn.py convert mnist-cnn-model
...
Model converted and saved to 'mnist-cnn-model.gguf'

$ ./mnist-cnn mnist-cnn-model.gguf models/mnist/t10k-images.idx3-ubyte