Add `Countable` instance for `gset`.

Thanks for the MR!

Cannot we put this in mapset.v? It probably makes most sense to put it below mapset_eq_dec there.

added 1 commit

Fixed. I had to import countable. Currently it is also exported. Let me know if you want me to change this to import only.

Export is fine, it's also exported in gmap after all.

Thanks & merging.

mentioned in commit 02a107a5

merged

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