feat(algebra/monoid_algebra): add add_monoid_algebra_ring_equiv_direct_sum (#7422)
feat(algebra/monoid_algebra): add add_monoid_algebra_ring_equiv_direct_sum (#7422)

The only interesting result here is:

    add_monoid_algebra_ring_equiv_direct_sum : add_monoid_algebra M ι ≃+* ⨁ i : ι, M

The rest of the new file is just boilerplate to translate `dfinsupp.single` into `direct_sum.of`.





leanprover-community/mathlib,leanprover-community/mathlib