Support MLP / neural network

[jonnor]

Is supported in emlearn. Just need to add some bindings for MicroPython.

Should probably also do a comparison to the other available alternatives available for MicroPython:

• https://github.com/jczic/MicroMLP
• https://github.com/mocleiri/tensorflow-micropython-examples
• https://medium.com/@subirmaity/a-simple-neural-network-implementation-approach-in-micropython-for-deep-learning-application-760ab35cb538
• https://gitlab.com/olivierlenoir/MicroPython-NeuralNetwork