Added Attean::IteratorSequence class.
Added Attean::API::Iterator->elements method.
Added Attean::API::Model methods get_list, get_sequence, subjects, predicates, objects, and graphs.
Added Attean::API::MutableModel->add_list method.
Updated Attean::API::BulkUpdatableModel to wrap model accessor methods to end bulk updates before the accessor reads any data.
Updated Attean::API::Parser roles to provide default implementations of all methods for Pull-, Push-, and AtOnce- parsing.
Updated Attean::MutableQuadModel->get_quads to provide union-term matching API.
Added Attean::QuadModel class by splitting immutable parts of Attean::MutableQuadModel.