access: read once what rules read at the same heads

bench_screening shows judging at about 17 µs of a change's 25: some
two and a half reads at its heads (the policy, bans, its node's member
entry, whether there is a registry, the sealed declarations), each
costing as much as working out what the heads cover. Screening judges
most of a batch at the same heads, and those reads are the same for
every change judged there.

Keep them (rules::Reads), by node and by user where they depend on
one, while the heads stay the same; screening keeps one for a batch.
What the heads hold doesn't change as the document takes changes, so
nothing goes stale but by other heads. The benchmark breakdown shows
judging so and afresh.

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