Implement the simple type checking rules

[snowmantw]

At this moment introducing complete but complex type checking rules isn't proper. We should implement a simple enough but useful type checking mechanism to prevent some obvious but hardly to discover errors, like asking for Context but receiving a function.

The main idea is to avoid to distinguish inherited objects like UI vs. IO, and check only primitive types like Function vs. Object, or "our primitive types" like Context vs. UI (different level).

This feature can be tough work, but it can show how important our "definition" stages are.