dev: span utility improvements

[lgritz]

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• Added as_bytes and as_writable_bytes that convert spans of arbitrary type to spans of std::byte. (This mimics C++20 utilities of the same name.)

• spancopy, spanset, spanzero: add parameter defaults for the common case of wanting to do the operation on the whole span, also simplify the bound-limiting logic in those functions a bit (but still performing precisely the same task).