[Merged by Bors] - feat(set_theory/cardinal/finite): prove lemmas to handle part_enat.card

[AntoineChambert-Loir]

Prove some lemmas that allow to handle part_enat.card of finite cardinals, analogous to those about nat.card

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[fpvandoorn]

LGTM

bors merge

[bors[bot]]

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