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.

Also, use coerce-pathname in asdf-ecl and deprecate merge-component-name-type.

Fix link in webpage (lp#614806).
Include SCL and ABCL in my testing routine.

When trying to explore the behavior of ASDF2 in connection with
rewriting that involves Lisp files, I was testing to verify that
ASDF will notice if we lose a .lisp file.  It does.  This test
checks that.