A proof-of-concept which embeds micropython in a max external

Currently only tested on macOS.

Can be built using -DBUILD_MICROPYTHON_EXTERNAL which will default to local micropython code. If the option -DFETCH_MICROPYTHON is used cmake will try to build from git clone.

Doesn't do anything now useful except prove that embedding micropytho in an external is possible.. Still a work-in-progress and will be developed further as one familiarizes oneself with the micropython c-api.