Add `flip` version of `Proper` for entailment of `wp`.

We have these instances for all other logical operations too to support
setoid rewriting in both directions.
6 jobs for master in 15 minutes and 18 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #41226
fp
build-coq.8.10.dev

00:12:50

passed #41230
fp
build-coq.8.7.2

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passed #41229
fp
build-coq.8.8.2

00:06:48

passed #41228
fp-timing
build-coq.8.9.0

00:06:09

passed #41227
fp
build-coq.8.9.1

00:07:21

passed #41225
fp
build-coq.dev

00:15:18