fix(zip): cleanup/fix of StringCodec

[piksel]

This PR fixes some issues that were present in #592 but wasn't detected before the release of v1.4.0.

In general, it goes a bit further in trying to make the string encoding easy and predictable to use. It also has tests for the most common usages of the API to make sure it both works and feels straightforward to use. The changes in the API are slightly breaking from v1.4.0, but only where the API could be misused in confusing ways. It should not require any minor version bump, but should be released ASAP to limit the effect.

Fixes #777. Fixes #776.

I certify that I own, and have sufficient rights to contribute, all source code and related material intended to be compiled or integrated with the source code for the SharpZipLib open source product (the "Contribution"). My Contribution is licensed under the MIT License.

[filipnavara]

Any news on this one? It's blocking us from updating to the latest NuGet.

[piksel]

Sorry, I have been busy with work. There are some inconsistencies in the API that I need to look over, but it should mostly be done.

[codecov[bot]]

Codecov Report

Merging #778 (4f90718) into master (d2a0c68) will increase coverage by 0.01%. The diff coverage is 75.47%.

@@            Coverage Diff             @@
##           master     #778      +/-   ##
==========================================
+ Coverage   74.70%   74.72%   +0.01%     
==========================================
  Files          72       72              
  Lines        8465     8485      +20     
==========================================
+ Hits         6324     6340      +16     
- Misses       2141     2145       +4     

Impacted Files	Coverage Δ
...ib/Zip/Compression/Streams/DeflaterOutputStream.cs	81.00% <50.00%> (-1.48%)	:arrow_down:
src/ICSharpCode.SharpZipLib/Zip/ZipStrings.cs	46.66% <65.38%> (-2.18%)	:arrow_down:
src/ICSharpCode.SharpZipLib/Zip/ZipFile.cs	77.38% <77.77%> (+0.13%)	:arrow_up:
src/ICSharpCode.SharpZipLib/Zip/FastZip.cs	59.51% <100.00%> (+0.34%)	:arrow_up:
src/ICSharpCode.SharpZipLib/Zip/ZipFormat.cs	89.54% <100.00%> (ø)
src/ICSharpCode.SharpZipLib/Zip/ZipInputStream.cs	83.02% <100.00%> (+0.56%)	:arrow_up:
src/ICSharpCode.SharpZipLib/Zip/ZipOutputStream.cs	87.86% <100.00%> (+0.04%)	:arrow_up:
src/ICSharpCode.SharpZipLib/GZip/GZipConstants.cs	50.00% <0.00%> (-50.00%)	:arrow_down:
... and 1 more

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