Newer version of RedBlackStencil.hs

Inlined two nested smaps under a szipWith using new fn overRSum
This allowed removal of lets in relaxLaplace
Also allowed over-relaxing lines to be moved to before boundMask/boundValue setting (either order is ok).

Very small change in performance.
But now can simply comment out the over-relaxation lines to discover they are not a significant problem.
Therefore altMapStencil2 must be slower than it should be.