Reimplement .compare() in terms of a visitor

Reimplement .compare() in terms of a visitor

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