Work list of C# 7 new syntax

[sharwell]

I'll be creating a separate issue for each of the following new syntax items in C# 7 so we can review its impact on all of the current rules.

• Tuples
• Deconstructors
• Pattern matching with is expressions
• Pattern matching with switch statement
• Local functions (Add tests for local functions #2371)
• Return by reference
• Out variables
• Literal improvements
  • Digit separators
  • Binary literals
• Generalized async return types
• More expression-bodied members
  • Constructors
  • Finalizers
  • Property accessors
• Throw expressions