Introduce local caching of "this" in async methods

Adds new API for tuple elements.

Update the compiler in the Linux/Mac toolset to support C# 7 features.

Move the ImportAdder over to using the IAddImportsService.

Updates the compiler used in the Linux and Mac toolsets and also
eliminates the requirement to download the toolset as a zip and instead
restores the necessary components over NuGet.

Produce more fine-grained information about what a token means.

Fixes #3589

Address outstanding issues around custom modifiers in C#.

Reduce allocations when creating syntax items.