Add APIs for working with JS types

[JakeWharton]

ArrayBuffer, UInt8Array, etc.

[swankjesse]

Yes!

[JakeWharton]

Probably

• BufferedSink.write(ArrayBuffer) + offset/count
• BufferedSource.readArrayBuffer + count overload
• ArrayBuffer.toByteString
• ByteString.asArrayBuffer (or "to"?)

Currently just cheating with:

private fun ArrayBuffer.toByteString(): ByteString {
  val uint8Array = Uint8Array(this)
  val buffer = Buffer()
  for (i in 0 until uint8Array.length) {
    buffer.writeByte(uint8Array[i].toInt())
  }
  return buffer.readByteString()
}