mathlib
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feat(data/finsupp/equiv_dfinsupp): add an equivalence between finsupp and dfinsupp
Doing this for subgroup is slightly harder as some simp lemmas are missing - so I will leave that to a follow up.
- [x] depends on: #7311
The missing simp lemma that would be needed for subgroups is in #7218
This PR/issue depends on:
- ~~leanprover-community/mathlib#7311~~ By Dependent Issues (🤖). Happy coding!