* Changing DOTNET35 framework symbols in preprocessor directives to the default built-in value of NET35.
* Adding extension method StreamExtension.CopyTo for .NET 3.5 because it didn’t exist until .NET 4, and adding associated unit tests.
Fixes#2088.
We now have separate tests for netcoreapp and net45 to test the two branches here.
(netstandard10 doesn't have MemoryStream.GetBuffer)
Although most of this library doesn't have any async functionality,
this feels like a natural place to locally add it.
John Brock
Jon Skeet
Jan Tattermusch
Jon Skeet