Add Olo

This defines Ordered Lists as effectively a more ergonomic kind of
Aggregation, which helps with bridging them and Ore‐ or P·C·D·M‐based
models. One caveat is that this requires Slots to have exactly one
item, and their item to be an object (i·e not a literal). In an
alternate universe, Slots could instead have had _either_ an item _or_
a value, but it’s simple enough to just make their item be an object
with that value instead.