std.zip: Replace magic constants by constants named after specs

[ghost]

This PR removes three public items: digiSignLength, eocd64LocLength and eocd64Length. They are replaced by private enums with names taken directly from the specs, making all names consistent.

Theoretically they should go through a deprecation process. But it's hard to imagine a usecase, where someone used these constants outside of std.zip. In my oppinion they should have been private from the very beginning.

[dlang-bot]

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