Persistent storage support

I haven't found a way to access the persistent storage in micro:bit with this crate (I may be blind though).

It's exposed in the [MicroPython](https://microbit-micropython.readthedocs.io/en/latest/filesystem.html) and [micro:bit runtime](https://lancaster-university.github.io/microbit-docs/ubit/storage) APIs.

I think it would be cool to support this here as well.