2.26.82: much cleanups and fixes on the driver.
Tests: SBCL passes test-lisp and test-upgrade Also: * big refactoring of test infrastructure * contrib/debug.lisp and (asdf/driver:asdf-debug) for debugging. * remove aif and it, use the cleaner if-bind. * backtrace support, with improvements from trivial-backtrace. * integration of asdf-condition-control (originally from xcvb-driver) * simplify the upgrade heuristic: always tries to upgrade once. The previous heuristic was too complex with too many failure cases. You are thus assumed to want a given version asdf if and only if it is in your source-registry.