- "primary mapper entity" is now an attribute on Query,
making _MapperEntity slightly less dependent on a particular parent
Query (in theory more shareable by multiple Query objects in different contexts)
- remove some comments that have been misunderstanding what _mapper_entities
does, or perhaps forgot to get removed
- simplify _mapper_entities
- The :class:`.CreateColumn` construct can be appled to a custom
compilation rule which allows skipping of columns, by producing
a rule that returns ``None``.  Also in 0.8.3.