sequences: seq_iter_init walks records via prim_seq's kv decomposition #155

Job	Run time
ubuntu-24.04	1m 29s
windows-2022	48s
macos-14	51s
ubuntu-24.04-arm	1m 30s
external-test-suite (clojure-test-suite)	1m 8s
perf-gate (linux)	25s
	6m 11s

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