Add support for generating Python code for CPython and CircuitPython

[vkottler]

A runtimepy variant that generates a Protocol implementation should be reasonably straight-forward (for host-side messaging implementation).

For CircuitPython, use: https://docs.circuitpython.org/en/latest/docs/library/ctypes.html.

Does this mean we need to multiplex output types / support multiple output-file configurations (or would "Python" vs. "C++" selection be better?).

[vkottler]

Generating unit tests / unit-testing this behavior would be useful (probably impossible for CircuitPython? does https://circuitpython.org/blinka help with this?

[vkottler]

See also: vkottler/homestead#6.

[vkottler]

#66 can continue implementing business logic for Python code generation.

[vkottler]

Generate code/config for vkottler/runtimepy#262.

[vkottler]

Can now specify byte_order for any struct definition, we should integrate a byte order parameter (with a good default, network order since it's the default in runtimepy?)

would also make sense to generate C++ code that will perform the "default" byte swap action