Provide a way to read multiple bytes at once synchronously

[nex3]

Currently, dart:io only makes it possible to synchronously read from a RandomAccessFile or Stdin one byte at a time (or line-by-line, but that just calls readByteSync() internally). In cases where we know how much data we want, it would be nice to have a Uint8List readBytes(int length) method to read exactly that much data without having to ultimately call the C read() function over and over again.