Item 17 left two correctness items and a loose end. The loose end is the
answer to the first item: flan_vec_as_slice's Flan-level return type is Unit,
so is_void answers before is_agg is ever tested, and that is the convention
rather than one symbol's accident. So there was no sret-for-Rt to build, and
building one would have been untestable and wrong.
The second item is built and tested by the only thing that can test it, a
preloaded dlsym, because a dev build with nothing yet redefined prints what a
release build prints whether or not anything reads a cell.
Survey: 93 MATCH / 0 DIFFER / 2 refused before, 97 / 0 / 0 after; 97 / 0 / 0
again with --dev on both sides.
