implement support for __prepare__ declarative method. this signals that

implement support for __prepare__ declarative method.  this signals that
the mapping should not be constructed immediately; instead, the info for
the mapping is placed into an interm object _MapperThingy which serves
as the mapper for the purposes of figuring out inheritance, etc.  Later,
the prepare_deferred_mapping() function can be called given a base which
will invoke __prepare__ for each mapped class.