…eritance

Because the GenConverter specifically handles generating code for
attrs classes, and because it registers hooks that don't require
function dispatch, singledispatch was preventing subclasses from
having code generated for them, because they would trigger the
previously-generated base class's structure/unstructure code.

This changes MultiStrategyDispatch to allow for a class-based hook to
specifically avoid single-dispatch. Since we know that a given attrs
class can always have code generated for it, we don't really want to
share dispatch across subclasses - like the original Converter, we
want to make sure we're structuring each individual attrs class as its
own separate type.