skipUpTo helpfully falls back from skip invocations which return zero to invoke a read into a buffer.

The current implementation has two inefficiencies which cause unexpected performance degradation in tight loops, especially when a small number of bytes are requested:

1. In many cases skipSafely is able to skip all the requested bytes, there's no need to allocate a read buffer
2. The caller only requests a few bytes are skipped. Calling `ByteStreams.skipFully(stream, 32)` always allocates a 8192 byte buffer.

Proposed solution:
Defer buffer allocation until it is needed, using the minimum of the requested size and BUFFER_SIZE. This reduces allocation pressure in all cases of small skip values, as well as streams that report nonzero bytes available (assuming skip returns a positive value).
public int read(byte[] b, int off, int len) throws IOException {
      if (left == 0) {
        return -1;
      }
      len = (int) Math.min(len, left);
      int result = in.read(b, off, len);
      if (result != -1) {
        left -= result;
      }
      return result;
    }
    @Override
    public synchronized void reset() throws IOException {
      if (!in.markSupported()) {
        throw new IOException("Mark not supported");
      }
      if (mark == -1) {
        throw new IOException("Mark not set");
      }
      in.reset();
      left = mark;
    }
    @Override
    public long skip(long n) throws IOException {
      n = Math.min(n, left);
      long skipped = in.skip(n);
      left -= skipped;
      return skipped;
    }
  }