feat(tar): support for async streams

[nathan-c]

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.

Relates to #223

I have taken a first pass at making TAR support async streams which will help with streaming to and from network streams. I have also tried to dramatically reduce memory allocations associated with reading from and writing to TAR's.

I haven't added new tests yet or really checked what the coverage is like but if you are happy with how it looks as it, I can do that.

Below are some benchmarks run before and after my changes:
Before:

Method	Job	Toolchain	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
ReadTarInputStream	Job-IFGTKW	.NET Core 3.1	122.3 μs	1.71 μs	1.43 μs	33.2031	1.2207	-	1112144 B
ReadTarInputStreamAsync	Job-IFGTKW	.NET Core 3.1	1,081.9 μs	21.64 μs	58.50 μs	37.1094	0.9766	-	1227248 B
WriteTarOutputStream	Job-IFGTKW	.NET Core 3.1	50.54 μs	0.339 μs	0.283 μs	0.3052	-	-	12008 B
WriteTarOutputStreamAsync	Job-IFGTKW	.NET Core 3.1	1,135.09 μs	22.460 μs	51.154 μs	3.9063	-	-	126985 B

After:

Method	Job	Toolchain	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
ReadTarInputStream	Job-KKAMPY	.NET Core 3.1	221.3 μs	0.76 μs	0.64 μs	-	-	-	952 B
ReadTarInputStreamAsync	Job-KKAMPY	.NET Core 3.1	227.1 μs	4.05 μs	3.79 μs	-	-	-	1096 B
WriteTarOutputStream	Job-KKAMPY	.NET Core 3.1	149.7 μs	2.71 μs	2.54 μs	-	-	-	449 B
WriteTarOutputStreamAsync	Job-KKAMPY	.NET Core 3.1	146.7 μs	0.67 μs	0.56 μs	-	-	-	448 B

[codecov]

Codecov Report

Merging #746 (23e5f27) into master (cc8dd78) will increase coverage by 0.27%.
The diff coverage is 77.25%.

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
+ Coverage   73.83%   74.11%   +0.27%     
==========================================
  Files          69       71       +2     
  Lines        8355     8441      +86     
==========================================
+ Hits         6169     6256      +87     
+ Misses       2186     2185       -1     

Impacted Files	Coverage Δ
src/ICSharpCode.SharpZipLib/Tar/TarInputStream.cs	63.20% <62.88%> (+7.84%)	⬆️
src/ICSharpCode.SharpZipLib/Tar/TarOutputStream.cs	67.83% <67.44%> (+0.90%)	⬆️
...rc/ICSharpCode.SharpZipLib/Core/ExactMemoryPool.cs	75.00% <75.00%> (ø)
src/ICSharpCode.SharpZipLib/Tar/TarBuffer.cs	73.12% <86.66%> (+6.45%)	⬆️
src/ICSharpCode.SharpZipLib/Tar/TarHeader.cs	80.86% <95.83%> (+1.37%)	⬆️
.../ICSharpCode.SharpZipLib/Core/StringBuilderPool.cs	100.00% <100.00%> (ø)
src/ICSharpCode.SharpZipLib/Tar/TarEntry.cs	67.79% <100.00%> (-1.50%)	⬇️

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[piksel]

Wow, for some reason I thought Stream.WriteAsync was not available in .NET 4.5...
This should make the async versions of Zip[In/Out]putStream much clearer.

Anyway, this mostly looks great, but it does lower the test coverage a bit.

[piksel]

There should be some more tests with the async versions of the code, but otherwise it looks good. Great work!

[nathan-c]

Thanks for the feedback! I will create some more tests to cover the changes

[nathan-c]

Some extra tests have now been added.