Keep an event's lists in arrays, flat or at their keys, and read a field by key as one (021)

- pure_list_arrays: an event's list (never shared, journaled or undone)
  keeps its items in arrays: FLAT in order, or DENSE at their keys when
  keyed by small whole numbers. A position asked of a DENSE list makes it
  FLAT; moves past a few times its items, arrays past 64 MiB, or a third
  change of form make it a tree. Answers are the tree's; the stored bytes
  stand for its items in the memory budget whatever it keeps them in.
- pure_list_add need not say where an item went: a list no index state
  refers to is not asked.
- A LIST_FIND whose index the next ITEM_GET of its list takes at once is
  joined to it at load (PLAN_JOINED): pure_list_lookup reads the field by
  key, at the key in a DENSE list.
- test-module arrays_equivalence: beside a tree, every step of a random
  run (adds, inserts, removes, changes, finds, reads, runs, clears) gives
  the same answers through every form and change of form.
- A search (100,000 nodes, 1,000,000 edges, core only): 187 to 126 ms.

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