Tests: SBCL passes test-lisp and test-upgrade
* 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.