feat (analysis/normed_space/lp_space):add l_infinity ring instances (#14104)
feat (analysis/normed_space/lp_space):add l_infinity ring instances (#14104)

We define pointwise multiplication on `lp E ∞` and give it has_mul, non_unital_ring, non_unital_normed_ring, ring, normed_ring, comm_ring and normed_comm_ring instances under the appropriate assumptions.



Co-authored-by: Eric Wieser <da64e90baec5d084ce3d8e05f3b022ae6d118ff5@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib