Add `Countable` instance for `mapset`.

Status Job ID Name Coverage
  Build
canceled #15391
fp
build-coq.8.6.0

canceled #15390
fp
build-coq.8.6.1

canceled #15389
fp
build-coq.8.7.0

canceled #15388
fp
build-coq.8.7.1

canceled #15387
fp-timing
build-coq.8.7.2

canceled #15386
fp-timing
build-coq.8.8.0

canceled #15385
fp
build-coq.dev