Simplify Relationship::Iterator::Tuples#initialize_tuples
Decouple Relation::Graph::Node from AttributeSet

Initial step towards completely decoupling the
Relation::Graph namespace from anything mapper
related. The relation graph should be a layer below
the mapping layer. Eventually it should be possible
to construct a mapper registry based on an either
explicitly defined or automatically inferred
relation graph, just as it's currently possible to
construct a relation graph from a mapper registry.
The process of automatically inferring a relation
graph involves reading a database (information)
schema plus generating relationship definitions
based on (configurable) conventions.

This will involve decoupling Graph::Node::NameSet,
Graph::Connector and Graph::Connector::Builder from
MapperRegistry. To achieve that, relationship
definitions should be pushed down to Graph::Node,
leaving the mapper relationship definitions to be
just another DSL layer on top of relation graph
relationship definitions.

This will make it possible to use the relation graph
without the mapper, sporting full relationship
support. The relation graph and the mapping layer
will expose the same API. Without the additional
overhead of mapping to domain objects, this might
be a nice choice for reporting scenarios, for pure
performance reasons, or even for simply inspecting
a database with a nice, relational algebra API.