Functional memory cache implmenation

Here is a functional baseline cache implementation. At this point we are only establishing behavior and usage patterns and writing unit tests and samples to verify those patterns. Please limit any performance related comments limited to architectural level issues.

What's implemented:
- Set, TryGetValue, Remove, extensions
- Eviction callbacks
- Active triggers with callbacks
- Checking for time and lazy trigger expiration when retrieving items.

What not implemented:
- Background expiration detection for time or lazy trigger based expiration.
- Memory pressure monitoring or cleanup.
Is this the right exception? Presumably for `default` to happen something awful must have happened somewhere else, no?