`prod(x::Array)` is much faster, and handles zeros correctly. 
`prod(x::Array; dims)` now uses a keyword, and just uses `mapslices(∇prod, x; dims)`. 
`prod(x::AbstractArray)` falls back to earlier `circshift...` methods. 
It would be easy to make `prod(f::Function, x::Array)` use fast `∇prod` but I didn't see a neat way to fall back, so left it alone. 

`cumprod(x)` now has a gradient; previously this gave Array{TrackedReal}. 
`cumprod(x; dims::Int)` works by something like `mapslices(∇cumprod, x, p, Δ; dims)` which I cooked up. 
(This was the case I originally wanted all this for!)