Add `Countable` instance for `mapset`.

7 jobs for master
latest
Status Name Job ID Coverage
  Build
canceled build-coq.8.6.0 #15391
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canceled build-coq.8.6.1 #15390
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canceled build-coq.8.7.0 #15389
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canceled build-coq.8.7.1 #15388
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canceled build-coq.8.7.2 #15387
fp-timing

canceled build-coq.8.8.0 #15386
fp-timing

canceled build-coq.dev #15385
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