Had to merge CHANGES.markdown.  Adding stuff for HEAD to the top of
the CHANGES file isn't a good idea when it comes to releasing bugfix
versions.  Maybe CHANGES for HEAD ought to go in a separate file and
then get added when cutting a new release...?

Note this change is *not* cherry-picked from master, where in fact we
depend on NumInstances == 1.2.* and import only
Data.NumInstances.Tuple.  However, that means that we have removed
some exported instances (the Num, Eq, and Show instances for function
types) and hence have to release that change with a major version
bump.  So here on the 0.6 maintenance branch we just bump the upper
bound so the old API is maintained but using a newer version of
NumInstances.

Using the formulas given in
http://www.mpi-hd.mpg.de/astrophysik/HEA/internal/Numerical_Recipes/f5-6.pdf .

(cherry picked from commit 2ecf3d1)