System.ValueTuple now supports .NET Framework 4.7

# Summary

The NuGet package System.ValueTuple [has been updated](https://www.nuget.org/packages/System.ValueTuple/4.4.0) to support .NET Framework 4.7.

# Details

The NuGet package provides the `ValueTuple` types which are required for the C# tuple syntax. In .NET Framework 4.7 we've added the types directly to `mscorlib`. If you use both, the NuGet package, as well as .NET Framework 4.7 you'll end up seeing the same types multipe times. This results in issues like this one [reported on Stack Overflow](https://stackoverflow.com/questions/45339914/predefined-type-system-valuetuple-is-not-defined-or-imported-after-upgrading-t).

We've updated the NuGet package to type forward the tuple types on .NET Framework 4.7 to `mscorlib`. This unifies the types and thus allows you to consume other libraries and NuGet packages that depend on `System.ValueTuple` while still targeting .NET Framework 4.7.

# Discussion

For discussion, see https://github.com/dotnet/corefx/issues/23085