For the "external" compiler component tests, create a simpler, composable APIs, based on current CompilerAssert framework.

The goal is to abstract one simple API for interacting with compiler which would:

• Enable one to easier write new tests.
• Be extensible

Implementation details:

• Have abstract C#, F#, IL compilation and referencing behind one API.
• Have composable APIs by abstracting compilation units and results.
• Have simple compilation assertion framework.
• Support baseline, directory-based, project-based, and inline testing.