Return raw pointer for `etl::multi_span::operator->()`.

[dhebbeker]

Resolves #772.

• using operator*() in case getting the current element is changed one day
• using & to get the address

Simply returning p_value may break in case the internal type of p_value would change. This way it is more robust to changes.

[semanticdiff-com[bot]]

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[jwellbelove]

Fixed 20.38.4