Use div_ceil where appropriate.

[nnethercote]

Several places in the tutorial need to do a division that rounds up, for chunking operations. They use a form like n / k + (n % k > 0).

I'm used to seeing this kind of division done with (n + (k - 1)) / k. This is probably preferred because it avoids the % operation, which is typically expensive.

But Rust has a builtin for this: div_ceil. Even better.