2.26.8: Fix the long-standing bug of dependency changes not being propagated between systems.
https://bugs.launchpad.net/asdf/+bug/479522 This used to be a glaring bug for most and a dubious "feature" for a few. Those few can nowadays reproduce their "feature" cleanly using :force-not if they want. Refactor the TRAVERSE protocol to fix the issue, notably replacing the special *forcing* by a lexical. Also fix another subtle bug whereby an unchanged dependency in a forced system could be re-compiled after being loaded. Kill many obsolete comments. Do :force and :force-not more cleanly, and fix the case where it's T.