This potentially avoids compiler warnings.

* 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.

This also updates the version number to 3.0.0-beta4

Move to dotnet cli for building, and .NET Core (netstandard1.0) as target platform

This also updates the version number to 3.0.0-beta4

JSON tests fail, as we're not using OriginalNameAttribute yet.

We'll see what I've missed when CI fails...

... and some implementation changes to go with them.

This is mostly just making things internal instead of public, removing and reordering a bunch of code in CodedInputStream/CodedOutputStream, and generally tidying up.