Use SYSTEM-SOURCE-FILE instead; also use LOAD-ASD as appropriate.

Uncomment compare-dependency-op, actually used by quux's make-qres-dependencies.lisp.
NB: not actually tested. There may be more bitrot.

Add TODO item.

Add print-object infrastructure for constituents (used while debugging).

* Load-source-op planning is broken, especially in how it trickles to
 other systems you depend on. Use load-op for planning, but have it
 perform like load-source-op on our instrumented files.
* Fix :conc-name nil in defstruct

This commit is related to XCVB's change 85a7d660021b5175c131122ef21c2e334afc54ab, which updates XCVB to use ADG's fine-grain-instrumented-load when converting from asdf-to-xcvb.

  - Added instrumentation for #. and #S
  - Potentially dangerous heuristics for defvar and defconstant
  - Improved handler for defstruct
  - New unit tests
  - Some cleanup of old code

Reduce number of style-warning through re-ordering of dependencies.

* tweak whitespace in the Makefile
* add and document a *system-base-dir* variable.
* remove some debugging stuff

 * Better recognition of just about everything.
 * Lots of bug fixes.
 * Make it easier to drop in user-provided handler definitions in the
   handlers/ directory.