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.
And move test scripts into t/ subdir to keep things clean.
As part of that, make sure redo adds itself to the PATH on startup so that
subscripts can find it.
