feat(data/finset/basic) eq_of_mem_singleton (#10414)
feat(data/finset/basic) eq_of_mem_singleton (#10414)

The `finset` equivalent of [set.eq_of_mem_singleton](https://leanprover-community.github.io/mathlib_docs/find/set.eq_of_mem_singleton/src)



Co-authored-by: Iván Sadofschi Costa <3cd556df6c016758512187105a01befc6c47b00c@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib