arraymath muladd frule has too loose types

[Keno]

The frule here:

ChainRules.jl/src/rulesets/Base/arraymath.jl

Lines 213 to 216 in f0095e0

	function frule((_, ΔA, ΔB, Δz), ::typeof(muladd), A, B, z)
	Ω = muladd(A, B, z)
	return Ω, ΔA * B .+ A * ΔB .+ Δz
	end

is Any typed. This is in general undesirable, because an frule this wide may be worse than the frule that would have been generated using AD recursion. In particular, this rule catches ChainRulesCore's methods for muladd on ZeroTangent which would otherwise have been trivial. I'd be good to narrow these types.