improve ByteStrings slice method - #3428

override def slice(from: Int, until: Int): ByteString =
  if (from <= 0 && until >= length) this
  else if (from > length || until <= from) ByteString.empty
  else drop(from).dropRight(length - until)

override def slice(from: Int, until: Int): ByteString = {
  val lo = math.max(from, 0)
  val hi = math.min(until, length)
  if (lo >= hi) ByteString.empty
  else if (lo == 0 && hi == length) this
  else {
    // find start fragment and offset
    var fragIdx = 0
    var fragOff = lo
    while (fragOff >= bytestrings(fragIdx).length) {
      fragOff -= bytestrings(fragIdx).length
      fragIdx += 1
    }
    // collect fragments in range
    val builder = new VectorBuilder[ByteString1]
    var remaining = hi - lo
    var firstOff = fragOff
    while (remaining > 0) {
      val frag = bytestrings(fragIdx)
      val take = math.min(remaining, frag.length - firstOff)
      builder += (if (firstOff == 0 && take == frag.length) frag
                  else ByteString1(frag.bytes, frag.startIndex + firstOff, take))
      remaining -= take
      fragIdx += 1
      firstOff = 0
    }
    ByteStrings(builder.result(), hi - lo)
  }
}