Don't redundantly readIO/parse the rulesets; do this once, lazily
carrying out the involved operations. Rulesets are invariant over
executions.
This improves performance by a few orders of magnitude. Though at some
point we should substitute linear search for lookup on a generalised
suffix tree of rooted domains.
Breaking interface change, though I'll likely restore the old form
soon with IO TH.
As 'a6f28e07a1edc8f62f3dfaf7965b3a818c2f4a7f' showed, there may be
breaking changes in the structure of rulesets between releases. I
don't intend to verify that every pair in the product works (is there
reason to be interested in any other than the latest?), so let's not
acommodate any more than one.
vi
