Consider updating the Code type to match the existing Code type.

[jakemac53]

This would allow us to provide some more safety in APIs that accept code (accepting only certain subtypes of Code) and convenience around building certain types of code such as type annotations.

Ultimately being extension types we can't make this perfect especially in terms of the safety aspect, we can't rely on anything. But, it still puts people on a golden path that is harder to use incorrectly.