Versions of `elem_of_list_split` that give first or last element.

8 jobs for master in 18 minutes and 36 seconds
Status Name Job ID Coverage
  Build
passed build-coq.8.7.0 #20521
fp

00:02:53

passed build-coq.8.7.1 #20520
fp

00:02:56

passed build-coq.8.7.2 #20519
fp-timing

00:02:34

passed build-coq.8.8.0 #20518
fp-timing

00:02:44

passed build-coq.8.8.1 #20517
fp

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passed build-coq.8.8.2 #20516
fp

00:02:51

passed build-coq.8.9.dev #20515
fp

00:03:57

passed build-coq.dev #20514
fp

00:15:45