public static byte[] scalarMultWithBaseToBytes(byte[] a) {
    return scalarMultWithBase(a).toBytes();
  }
Computes {@code a}*B
where a = a[0]+256*a[1]+...+256^31 a[31] and
B is the Ed25519 base point (x,4/5) with x positive.

Preconditions:
a[31] <= 127
java_src/src/main/java/com/google/crypto/tink/internal/Ed25519.java