Consider #[track_caller] on functions that panic

[joshtriplett]

For functions that panic, such as extend, please consider using #[track_caller], to provide more meaningful location information for debugging such panics.

(This report inspired by debugging an ArrayVec panic inside rust-lang/rust CI.)

[bluss]

Needs knowledge about the performance impact - probably not a great concern for extend, but for other hotter methods.

[joshtriplett]

@bluss Absolutely. I filed this issue rather than submitting a PR, because this would absolutely require checking the performance impact.

[bluss]

Fixed by #236