dep: Major overhaul.

The previous implementation was just fundamentally incorrect.  The new
version is somewhat better organized (though fairly similar
superficially) and documented, and actually (I think) correct.

Changes include:

  * DELAY-RECOMPUTING-DEPS has been renamed to WITH-DEPS-FROZEN.  The
    new version is semantically slightly different: it's the only point
    which actually triggers recomputation.  It has also grown the
    ability to defer its body until after the current recomputation
    phase is complete.

  * It is now a (diagnosed) error to attempt to modify a dep during
    recomputation; modifications can be deferred explicitly, though
    doing this sort of thing automatically seems a bad idea.