seqZ: move definition and use different length lemma

Status Job ID Name Coverage
  Build
canceled #35539
fp
build-coq.8.10.dev

canceled #35545
fp
build-coq.8.7.2

canceled #35544
fp-timing
build-coq.8.8.0

canceled #35543
fp
build-coq.8.8.1

canceled #35542
fp
build-coq.8.8.2

canceled #35541
fp-timing
build-coq.8.9.0

canceled #35540
fp
build-coq.8.9.1

canceled #35538
fp
build-coq.dev