Distinguish byte-compiled files from MicroPython's #4703
Description
In #4697 a user mentioned using a version of mpy-cross built from micropython sources with CircuitPython. I don't think we ever guaranteed that the format of .mpy files was the same as our base version of micropython, though this has been true for a long time. I believe this will change with #4693, as we renumbered some of the flags in runtime0.h.
The .mpy file begins with two bytes: ASCII 'M', and then the bytecode version number (currently 4, will become 5 soon). I see two main ways to ensure we can distinguish our own files:
- Change 'M' to 'C', or
- Set the high bit in the version field (128|5 -> 133)
The purpose of the change would be to prevent users accidentally or intentionally bringing mpy files built with micropython's mpy-cross to circuitpython devices.
We could standardize on a ".cpy" extension at the same time, though this is likely to create a lot of tooling problems as well as making lots of updates in the learn system necessary.