add Result type

[jan-ferdinand]

• add Result type
• add Ok function
• add Err function
• support ? operator

fix #49

[Sword-Smith]

I realized that if we represent a Result as a DataType::Enum internally, then get match support and correct stack-management for free. We even get the constructor logic Result::Ok(a) and Result::Error(b) for free as well!

This can be handled already in the grafter, resolving Result to the correct DataType::Enum instance.