policy: T9379: prefix-list duplicate check in linear time

verify() looked up each prefix-list rule in a list of the rules seen
so far, which is quadratic in the size of the list and runs on every
policy commit. verify() alone, run from the source on a workstation,
took 41.8 s for a 40,000-rule list; on a 1.5.1 VM the check cost 0.08 s
at 2,000 rules.

Keep a set of the rules' sorted items instead. Every value under a
prefix-list rule is a single leaf, so the key is hashable, and two keys
are equal exactly when the rule dicts are: the same rules are flagged,
with the same message. The workstation benchmark took 0.11 s.

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