Tweaks to sum across dimensions

- Implement sum(fn, A, region)
- Implement Base.sumabs across dimensions
- Use pairwise summation/BLAS for sums across first dimension
tweaks of prod functions
Merge pull request #7040 from lindahua/dh/prod2

tweaks of prod functions
Merge pull request #7035 from JuliaLang/sjk/sum

Tweaks to sum across dimensions