This greatly reduces the number of fork+exec calls, so in particular,
t/curse/all.do now runs much faster:
/bin/sh (bash): was 5.9s, now 2.2s
/bin/dash: was 3.2s, now 1.1s
Obviously improving the speed of minimal/do doesn't really matter, except
that it makes a good benchmark to compare the "real" redo against. So far
it's losing badly: 5.4s.
This could be good for distributing with your packages, so that people who
don't have redo installed can at least build it. Also, we could use it for
building redo itself.
Will surely need to get slightly bigger as I inevitably discover I've
forgotten a critical feature.
