Expose `AsyncEnumerable` in netstandard2.0 #436

Job	Run time
csproj-build (Release, true)	59s
csproj-build (Release, false)	44s
csproj-build (Release_NoProgress, true)	56s
csproj-build (Release_NoProgress, false)	44s
csproj-build (Debug, true)	44s
csproj-build (Debug, false)	48s
csproj-build (Debug_NoProgress, true)	1m 3s
csproj-build (Debug_NoProgress, false)	1m 6s
	7m 4s

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