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

8 jobs for master
latest
Status Job ID Name Coverage
  Build
canceled #20811
fp
build-coq.8.7.0

canceled #20810
fp
build-coq.8.7.1

canceled #20809
fp-timing
build-coq.8.7.2

canceled #20808
fp-timing
build-coq.8.8.0

canceled #20807
fp
build-coq.8.8.1

canceled #20806
fp
build-coq.8.8.2

canceled #20805
fp
build-coq.8.9.dev

canceled #20804
fp
build-coq.dev