Consider splitting `Decor` out of types

[epage]

Right now, we have

• Value::* which is Formatted which has Decor and Repr
• Array which has Decor and Span
• InlineTable which has Decor and Span
• Key which has two Decors and Repr

This makes it annoying for operating on a Values decor because you have to handle every case differently. This also requires manual work to maintain the Decor when changing a Value.

Could have a pub Decoration type and pub Decorated<T>?

Would love to do something similar for Repr but we have to make sure that gets deleted when a Value is overwritten.