Breaking change to split Attribute out into its own Type.

Realistically, this should not break very much code, but it technically has the potential to require some changes.

Attribute used to be a typealias to TransformedAttribute with an IdentityTransformer. Now, Attribute is its own Type. Although it is a bit sad to see a new Type created where a typealias used to suffice, the benefit of Attribute being its own Type is that the use of it is much more concise. Instead of Attribute offering initialization and containing Type information only relevant to TransformedAttribute, it will not have a cleaner interface.