traits are shorter and easier to identify by using tags in a variadic
template
recursive traits can be automatically derived, which makes it much
easier to add new traits to be handed through the expression tree.
The serializer has partial specializations for all templates. It is a
good basis if the connector/database requires strings close to the
standard.
The interpreter is unspecialized (and uses a static assert to say so).
It is a good basis if the connector/database requires a different
interpretation, e.g. re-writing the expression tree.
The interpretable_t implements three methods for interpretation:
serializing with the sqlpp::serializer_context, serializing with the
database's serialization context and interpretation with the database's
interpretation context.
