Use usize over i32 where possible

[philpax]

The original code uses int liberally due to C's wonderful integer promotion. I think it's easier for us to understand and less troublesome to index arrays if we offer a uniform usize interface across the board, and only cast to/from i32 when required.

I made this change in my take and found it to be much less messier and easier to reason about (especially with maintaining non-negativity constraints!), but I'm not sure how much it would impact the implementation here.