static void shuffleState(final int[] state) {
    for (int i = 0; i < 10; i++) {
      quarterRound(state, 0, 4, 8, 12);
      quarterRound(state, 1, 5, 9, 13);
      quarterRound(state, 2, 6, 10, 14);
      quarterRound(state, 3, 7, 11, 15);
      quarterRound(state, 0, 5, 10, 15);
      quarterRound(state, 1, 6, 11, 12);
      quarterRound(state, 2, 7, 8, 13);
      quarterRound(state, 3, 4, 9, 14);
    }
  }
Computes the 20 ChaCha rounds as described in
https://datatracker.ietf.org/doc/html/rfc7539#section-2.3.
java_src/src/main/java/com/google/crypto/tink/aead/internal/ChaCha20Util.java