Functions as "Array Initializers"

[uniment]

(Related Discourse discussion)

This would be nice:

x = Array{Float64}(m, n) do (i, j)
    i+j
end
x == Float64[i+j for i=1:m, j=1:n] # true

If #48404 is approved, then this will work by sending a CartesianIndex as an argument.