Use type hint in pointer arithmetic

[ds5678]

Resolves #3411

Problem

This makes pointer arithmetic prettier with the help of available type hints.

Solution

• Any comments on the approach taken, its consistency with surrounding code, etc.
  • I tried to be as surgical as possible.
• Which part of this PR is most in need of attention/improvement?
  • There was a slight behavior change involving sbyte* vs byte*. I want to know if that's okay or not. Previously, byte* got picked because it's the default when things go wrong. However, now the type hint (which is often signed) prevents some of those situations.
  • I think the type hints could be better if they had sign information in more cases, but I was hesitant to change the code that sets the type hint.
• At least one test covering the code changed