assertPassable = val => {
  passStyleOf(val); // throws if val is not a passable
}
If there is already a PassStyleOfEndowmentSymbol property on the global,
then presumably it was endowed for us by liveslots with a `passStyleOf`
function, so we should use and export that one instead.
Other software may have left it for us here,
but it would require write access to our global, or the ability to
provide endowments to our global, both of which seems adequate as a test of
whether it is authorized to serve the same role as liveslots.

NOTE HAZARD: This use by liveslots does rely on `passStyleOf` being
deterministic. If it is not, then in a liveslot-like virtualized
environment, it can be used to detect GC.

@type {PassStyleOf}