Compare observed types to existing type annotations

[cleder]

Use cases:

• I want to know if my tests test for every allowed input type
• I want to know if any of my existing allowed types (e.g: Union[str, int]) are unused in practice and can be removed.

[carljm]

Sounds useful! This feature shouldn't be hard to implement; effectively we need to do an "apply" in "override all existing annotations" mode, and then diff the result with the original file.

I suggest monkeytype diff <modulename> as the CLI.

PR welcome!