feat(zip): make it possible to skip header tests

[piksel]

This allows for skipping (some of) the local header tests performed when reading ZipFile entries.

Example:

using (var zipFile = new ZipFile(inputFile)){
	zipFile.SkipLocalEntryTestsOnLocate = true;
	foreach (var entry in zipFile)
	{
		// ...
	}
}

Also included are some refactoring done when investigating this. Mostly to make the code path easier to follow and provide better errors when things go wrong.

[codecov]

Codecov Report

Merging #689 (fc92777) into master (aee3b44) will decrease coverage by 0.01%.
The diff coverage is 60.46%.

@@            Coverage Diff             @@
##           master     #689      +/-   ##
==========================================
- Coverage   73.83%   73.82%   -0.02%     
==========================================
  Files          69       69              
  Lines        8355     8354       -1     
==========================================
- Hits         6169     6167       -2     
- Misses       2186     2187       +1     

Impacted Files	Coverage Δ
src/ICSharpCode.SharpZipLib/Zip/ZipConstants.cs	33.33% <0.00%> (-16.67%)	⬇️
src/ICSharpCode.SharpZipLib/Zip/ZipFile.cs	77.24% <61.90%> (-0.04%)	⬇️

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