can have errors when we do the auto-join, these are both covered in test_joins

fundamental and general purpose heuristic.   this initial approach
has about 60 tests failing but seems to have gone pretty far

options incorrectly, thereby breaking subsequent usage of the
:meth:`.Query.execution_options` method.  Courtesy Ryan Kelly.
[ticket:2661]

:func:`.aliased` construct without it interfering with the entities
being selected.  [ticket:2635]

- paths now store Mapper + MapperProperty now instead of string key,
so that the parent mapper for the property is known, supports same-named
properties on multiple subclasses
- the Mapper within the path is now always relevant to the property
to the right of it.  PathRegistry does the translation now, instead
of having all the outside users of PathRegistry worry about it,
to produce a path that is much more consistent.  Paths are now
consistent with mappings in all cases.  Special logic to get at
"with_polymorphic" structures and such added also.
- AliasedClass now has two modes, "use_mapper_path" and regular;
"use_mapper_path" is for all those situations where we put an AliasedClass
in for a plain class internally, and want it to "path" with the
plain mapper.
- The AliasedInsp is now the first class "entity" for an AliasedClass,
and is passed around internally and used as attr._parententity
and such.  it is the AliasedClass analogue for Mapper.

- it appears we can get rid of all those "XYZ_toplevel" names and use :doc:.