Add Kruskal-Wallis ranking and Kruskal-Wallis Test
`kruskalWallis` now ranks samples
Add `kruskalWallisTest`
Add more Kruskal-Wallis tests
Add documentation for Kruskal-Wallis module
implement Kendall's tau
add documentation, remove dependency on monad-primitive
fix indentation
add tests for Kendall, and fix one bug
add special cases test for Kendall
Small changes

 - Inline square function
 - Improve readability
Improve readability a somewhat

N.B. current implementation of mean uses Welford algorithm which
is less numerically robust than KBN summation it's more readable
Add module header
Merge branch 'kruskal-wallis'
Remove some unfortunate uses of INLINE
Drop a ton more uses of INLINE

--HG--
extra : amend_source : c838454f332970db35c7bad687558e146ae6043e
Deduplicate definitions of square
Get rid of obsolete GHC clauses
Small fixlet of null check
Drop an unneeded import