What's new in C# 7

[BillWagner]

This issue tracks the content for https://github.com/dotnet/core-docs/csharp/csharp-7.md

This topic will likely generate several sub-topics, one for each feature.

It should include a sample, or set of samples that demonstrate the new features coming with the C# language.

An initial list of C# 7 features is in this blog post by @MadsTorgersen: https://blogs.msdn.microsoft.com/dotnet/2016/08/24/whats-new-in-csharp-7-0/