Sum zero-arrays

[mcabbott]

Closes #362

Does not handle keywords, note that dims is an error on normal Arrays, but init is respected so perhaps should be here too:

julia> sum(fill(3), init=99)
102

julia> sum(fill(3), dims=1)
ERROR: BoundsError: attempt to access Tuple{} at index [1]

CI failure for GPU seems to be printing code?

[maleadt]

Good to go?

[mcabbott]

Sadly not -- status when last I looked was that my "better way" seemed not to be called at all, and I wasn't sure why.