feat(data/fintype/basic): add lemmas about finsets and cardinality (#5886)
feat(data/fintype/basic): add lemmas about finsets and cardinality (#5886)

Add lemmas about finsets and cardinality. Part of #5695 in order to prove Hall's marriage theorem.

Coauthors: 47c76fa59605c7630041cc7bf5f61d35238b9ee2@b-mehta 





leanprover-community/mathlib,leanprover-community/mathlib