Type Layout -> Layout; Make values primary

[Havvy]

Values have a defined layout, but types don't always have defined layout.

E.g. dynamically sized types have dynamic types and where padding is located depends on which enum discriminant is active.

As such, we should rename the type layout chapter to just layout and describe it as properties of values and then state that types may have defined properties if all values of the type have the same value for that property.