DeclarationList knows how to generate an attribute
list, which never really made sense outside of being generated from
declarations. Now, the declaration list builds a list of attributes
which is combined in Factory#attributes with attributes from traits and
its parents.
Both Factory and Trait have similar methods and interact with a
DefinitionProxy. The idea here is to move the interface DefinitionProxy
expects to a separate class and both Factory and Trait can delegate to
an instance of Definition.
