function writeStringOrBuffer (stream, toWrite) {
  if (toWrite && typeof toWrite === 'string') writeString(stream, toWrite)
  else if (toWrite) {
    writeNumber(stream, toWrite.length)
    stream.write(toWrite)
  } else writeNumber(stream, 0)
}
writeStringOrBuffer - write a String or Buffer with the its length prefix

@param <Buffer> buffer - destination
@param <Number> pos - offset
@param <String> toWrite - String or Buffer
@return <Number> number of bytes written