Separate boostrap definition from its evaluation
Experiment with step-big

With this change, building std-0-output takes less than one third the
time it did after adding explicit substitution.  It's interesting to
note that without explicit substitution, step-big doesn't bring such a
dramatic improvement.

Build time summary (seconds):
  base                   = 7.8
  base         + stepbig = 7.3
  base + exsub           = 3.5
  base + exsub + stepbig = 1.1
Add step-big interaction