Various cleanup

[Darksonn]

This performs various cleanup:

• Factor a lot of panics into functions to improve error messages and reduce codegen.
• Add missing #[inline] and #[track_caller].
• Fix some arithmetic overflows in bounds checks.

[hds]

The changes look good to me. The unsafe section explanations all appear correct. Although I'm not very familiar with this code base, so you might want someone else to have a look too.