Generalize `gset_to_propset` to `set_to_propset` for any SemiSet.

This closes issue #25.
8 jobs for master in 8 minutes and 4 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #30025
fp
build-coq.8.7.0

00:02:37

passed #30024
fp
build-coq.8.7.1

00:02:36

passed #30023
fp
build-coq.8.7.2

00:02:36

passed #30022
fp-timing
build-coq.8.8.0

00:02:32

passed #30021
fp
build-coq.8.8.1

00:02:45

passed #30020
fp
build-coq.8.8.2

00:02:46

passed #30019
fp-timing
build-coq.8.9.0

00:03:06

passed #30018
fp
build-coq.dev

00:08:00