Move common infrastructure to a separate assembly

[MikhailArkhipov]

We have bunch of common code in the engine, it is worth creating something similar to Microsoft.Common.Core in RTVS. Perhaps Microsoft.DSTool.Core. And copy tests for common stuff like CountdownDisposable etc to this repo.

[MikhailArkhipov]

Needed before AST analysis work. Also includes splitting analysis into Core, Parser, AST and DDG analysis components so we can start building new one without touching existing code.

[MikhailArkhipov]

#465