SortedSet find methods improvements

Fixes `findGreatestLessThanOrEqual`, `findGreatestLessThan`, `findLeastGreaterThanOrEqual`, and `findLeastGreaterThan` by acknowledging that `splay` doesn't always leave the right node on top when `splay`ing a value that isn't in the set. Fixes https://github.com/montagejs/collections/issues/26.