Mixins revision.

* Alternate more powerful implementation.
  * No need to pre-specify allowed extension points.
  * If two mixing provide the same method, or a single mixing and the main method, then a coordination method is always generated.
  * Handles the special preExecution case.
  * Can handle any case by overriding conventionally named methods "\_createMixinMethod\_&lt;forMethodName&gt;".
* Cleanup of the "legacy" AddIns code.
* Passed the tests on the first write up! (doesn't mean it will actually work...)
* Placed some notes here and there, for things I think should be fixed.

@pamval please appreciate.