Prevent underflow when using a MockI2CDevice with a RegisterMap.offset = 0x0

[chiefnoah]

Cast both the offset and the slice.len() to isize so we can safely underflow to a negative number. We lose out on 1-bit of possible offset/slice size, but I'd love to have your computer if you're bumping into that.

Adds a quick test to verify the fix as well.

This fixes #77.

[eldruin]

Thank you!
Could you rather do the conversion via: isize::try_from(self.offset).unwrap_or(0xBAD) or similar?

It is not very important because it is just a print in a mock, but it would be nicer to see "BAD" rather than "FFFFFFFFFFFFFFFF".

[chiefnoah]

How does that look?

[eldruin]

Looks good to me, thanks!
Could you also add an entry to the changelog?
Could you also raise the MSRV to 1.65.0 and note it in the changelog as well?

[chiefnoah]

Done.

[eldruin]

Great, thank you!