mean() can easily overflow for general iterables

[nalimilan]

Contrary to the mean(::AbstractArray) method, the mean fallback for general iterables computes the sum without promoting the element type, meaning that overflow can easily happen:

julia> mean([typemax(Int8), typemax(Int8)])
127.0

julia> mean(x for x in [typemax(Int8), typemax(Int8)])
-1.0

Apart from being inconvenient/risky, it's inconsistent with sum:

julia> sum([typemax(Int8), typemax(Int8)])
254

julia> sum(x for x in [typemax(Int8), typemax(Int8)])
254

I think we should use promote_sys_size_add added by #22825 to choose the accumulation type, just like sum.

[JeffreySarnoff]

makes sense to me