Add I2C and GPIO resource descriptors

[dewyatt]

No description provided.

[martin-hughes]

Thanks for this PR as well @dewyatt - it looks fundamentally sound to me.

Just a few nitpicks really, and a couple of questions for @IsaacWoods.

[martin-hughes]

@IsaacWoods should we be comparing the length of the descriptor buffer slices against the value of the length field contained within that slice?

I know there's no memory safety risk of skipping this check, it'd mostly be to avoid an out-of-bounds panic.

[IsaacWoods]

Yes I think the ideal would be for parsing resource descriptors to be entirely panic free (I can't recall if that's the position we're in atm)

[IsaacWoods]

Thanks @dewyatt for the PR and to @martin-hughes for review. Very happy for this to be merged from my end when questions addressed :)