SI-8589 Performance improvement for ArrayCharSequence.toString

While using a csv parser to process large char arrays i ran into the problem that the toString method of ArrayCharSequence was very slow on large buffers (and, as it appears, also on small buffers)
The problematic method:
override def toString = xs drop start take length mkString ""

I created a faster version which is already almost 50% faster on an empty charsequence and only becomes relatively faster when the array gets bigger.

See https://issues.scala-lang.org/browse/SI-8589