psplay refactoring

[ct]

the probabilistic splay tree implementation could benefit from some small refactorings (additional to the mpool).

• The probability formula is rather glued together by some guesswork (its not critical, thus works) but can be improved on some more solid statistic foundation.
• refactor the splay function to pass the common parameters (trail, factor, splayroot) as struct around. This shall allow more lightweight splay trees (generate the 'PSplay' struct on the fly) and working on subtrees.