void multByZ(long[] output, long[] in) {
      System.arraycopy(in, 0, output, 0, Field25519.LIMB_CNT);
    }
Creates a cached XYZT with Z = 1

@param yPlusX y + x
@param yMinusX y - x
@param t2d 2d * xy
java_src/src/main/java/com/google/crypto/tink/internal/Ed25519.java