I am enjoying these changelog edits
Improve Arbitrary instances
encodeUtf8_2: fix parallel range check

This makes it rather expensive, alas.
encodeUtf8_2: drop parallel range checks

Once I noticed that I'd screwed up the range checking and fixed it,
it became slow enough to not be worth it. All test cases are about
10% faster with this extra complexity removed, with the exception
of pure Russian, which is about 50% slower.