[ENH] Associations Please review and read the tests ( [er][su] passes tests on clean Squeak3.8g-6548.image )

[rastaehli at mac.com]

Despite the obscurity of some of the LookupKey subclasses, this is a
straightforward refactoring to improve documentation and fix obvious
(see Andrew's test suite) bugs in the old classes.  The most worrisome
aspect of the change set is the delicacy of converting all instances of
the old classes to the new.  The comments in the changeset make the
strategy clear enough, though I could not follow all the details. 
Testing on a clean Squeak3.8g-6548.image shows 1334/8/3
passed/failed/errors before the attached changes sets are loaded and
1349/8/3 after, indicating that all Andrew's new tests pass and that
none of the old tests break.