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This is far too useful in testing to reproduce in each package that
needs something like it.  It's already shown up as desirable again
as soon as I start implementing even a little bit of even one codec
tokenizer, and that's gonna keep happening.

This might be worth moving to some kind of a 'tokentest' or
'codectest' package instead of cluttering up this one, but...
we'll see; I've got a fair amount more code to flush into commits,
and after that we can reshake things and see if packages settle
out differently.