Add `proj1_ex` and `proj2_ex` when ∃ over `Prop`.

8 jobs for master in 10 minutes and 40 seconds (queued for 6 minutes and 50 seconds)
Status Job ID Name Coverage
  Build
passed #34953
fp
build-coq.8.10.dev

00:08:02

passed #34959
fp
build-coq.8.7.2

00:02:39

passed #34958
fp-timing
build-coq.8.8.0

00:02:35

passed #34957
fp
build-coq.8.8.1

00:03:04

passed #34956
fp
build-coq.8.8.2

00:02:52

passed #34955
fp-timing
build-coq.8.9.0

00:03:21

passed #34954
fp
build-coq.8.9.1

00:03:45

passed #34952
fp
build-coq.dev

00:08:43