[Haskell] Principal variation search

[Rene de Visser]

I've added some code of mine for principal variation search in haskell to 
the wiki

 http://www.haskell.org/haskellwiki/PrincipalVariationSearch

It might need a couple of strictness annotations for best performance, but 
it worked ok for me as is, when compiled with -O2 on GHC.

It is fairly basic (lacks support for hash tables for example).

But most other things can be done by generating the move tree lazily in the 
right way (example move ordering).

Rene.