SI-6642 Make mutable.TreeSet projection iterator O(log n)

Improves the efficiency of getting an iterator on a ranged projection
of a mutable.TreeSet O(log n) instead of O(n) by using the bounded
iterator mechanism on RedBlackTree instead of dropWhile.

A little more efficiency is gained from using pattern matching instead
of map/getOrElse to apply an upper bound to the iterator.

All the above leaves contains the only consumer of isLeftAcceptable and
isRightAcceptable, so they're moved inside the body of contains.

All this functionality is covered by
test/files/run/mutable-treeset.scala