v0.59.0

A floor under each layer

The coverage walk landed eight releases ago and asserted nothing at all. It
printed a number and a list, and a number nobody is accountable to is a number
that drifts.

It has floors now, and the shape of them is the decision worth explaining:
they are per layer, not one for the whole file. A single figure for 12478
lines lets a machine backend go quietly dark behind a hundred lines of
something easy to run somewhere else, and the average would never move. Seven
layers carry one, and they are the ones where being wrong is silent: the LLVM
lowering, the three machine backends, the wasm encoder, the module read back,
the machines read back, and the tier that asks for nothing underneath it. Each
floor is the number that layer reaches today, less a little.

The rest carry none, on purpose. The scenery does not need one, because being
wrong there is loud. And the coverage layer has none because it does not
measure itself, which was written down when it landed and is still true.

One thing the floors turned up before they were even set. The other language's
compilers had a layer sitting at 23 per cent, and the reason was not that they
went unchecked: it was that nothing in the exercise list called the thing that
checks them. Adding it took that layer to 86, and the file overall to 87.8.

python3 ouroboros.py --coverage

Full Changelog: v0.58.0...v0.59.0

LOC: 26060