SI-6642 Make mutable.TreeSet#count O(log n) worst case

Because RedBlackTree trees carry their size there was no need for
mutable.TreeSet#count to resort to a O(n) size implementation on
ranged projections. This commit improves the counting algorithm to take
advantage of the tree's count field and the tree's structure to
count nodes in range in O(log n) worst case time. Existing tests
already cover the functionality of mutable.TreeSet#size. Especially
see test/files/run/mutable-treeset.scala.