This leverages the work started in #4336 to allow ClauseElement
structures to be cachable based on structure, not just identity.
Change-Id: Ia99ddeb5353496dd7d61243245685f02b98d8100
Reworked the :meth:`.ClauseElement.compare` methods in terms of a new
visitor-based approach, and additionally added test coverage ensuring that
all :class:`.ClauseElement` subclasses can be accurately compared
against each other in terms of structure. Structural comparison
capability is used to a small degree within the ORM currently, however
it also may form the basis for new caching features.
Fixes: #4336
Change-Id: I581b667d8e1642a6c27165cc9f4aded1c66effc6
Mike Bayer