Use 5-arguments mul!

[lostella]

Some algorithms code may be improved (and maybe sped up?) by using the 5-arguments version of mul! (requires Julia 1.3), see https://docs.julialang.org/en/v1/stdlib/LinearAlgebra/#LinearAlgebra.mul!

For example, the code here could probably use it: https://github.com/JuliaFirstOrder/ProximalAlgorithms.jl/blob/9fe03c134bdea35a399450966be0cdc09714bcf2/src/algorithms/primal_dual.jl#L171