- created a link between QueryContext and SelectionContext; the attribute

- created a link between QueryContext and SelectionContext; the attribute
dictionary of QueryContext is now passed to SelectionContext inside
of Query.instances(), allowing messages to be passed between the two stages.
- removed the recent "exact match" behavior of Alias objects, they're back to
their usual behavior.
- tightened up the relationship between the Query's generation
  of "eager load" aliases, and Query.instances() which actually grabs the
  eagerly loaded rows.  If the aliases were not specifically generated for
  that statement by EagerLoader, the EagerLoader will not take effect
  when the rows are fetched.  This prevents columns from being grabbed accidentally
  as being part of an eager load when they were not meant for such, which can happen
  with textual SQL as well as some inheritance situations.  It's particularly important
  since the "anonymous aliasing" of columns uses simple integer counts now to generate
  labels.