Add `#[track_caller]` judiciously

[bluss]

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Add #[track_caller] to some panicking methods. Slicing methods seem the most prudent (Index/Mut does not apply, sorry).

Needs benchmarks to investigate impact, preferably holistic benchmarks.

[xd009642]

I started work on this in https://github.com/rust-ndarray/ndarray/pull/975