Add update() support for partial data in input buffer

[rustyx]

This makes it easier to update() from a data buffer, i.e. when reading from a stream.

        Nilsimsa nilsimsa = new Nilsimsa();
        byte buf[] = new byte[4096];
        for (int len = maxlen; len > 0;) {
            int bytesRead = content.read(buf, 0, Math.min(buf.length, len));
            if (bytesRead < 0)
                break;
            nilsimsa.update(buf, 0, bytesRead); // <=== LOOK!
            len -= bytesRead;
        }