public TrustAnchor index(X509Certificate cert) {
        TrustAnchor anchor = new TrustAnchor(cert, null);
        index(anchor);
        return anchor;
    }
Indexes {@code TrustAnchor} instances so they can be found in O(1)
time instead of O(N).
common/src/main/java/org/conscrypt/TrustedCertificateIndex.java