Fix examples that use static_mut

[9names]

Some the examples use static_mut:
I think we can definitely fix the IRQ ones but my naive attempts to resolve it for USB were not working.
Maybe the easiest solution there is to convert those examples to use RTIC instead?
We should resolve this before 2024 edition becomes a thing, so that the code will work across editions.

Originally posted by @9names in #101 (comment)