jni: Use only Guava for hex encoding and decoding
jni: Use only Guava for hex encoding and decoding

This removes a dependency on javax.xml.bind, which is no longer
available in JDK >= 11, see
https://openjdk.java.net/jeps/320#Java-EE-modules .

evoskuil/secp256k1,libbitcoin/secp256k1,pmienk/secp256k1,evoskuil/secp256k1,afk11/secp256k1,evoskuil/secp256k1,peterdettman/secp256k1,pmienk/secp256k1,bitcoin/secp256k1,evoskuil/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,libbitcoin/secp256k1,pmienk/secp256k1,afk11/secp256k1,libbitcoin/secp256k1,peterdettman/secp256k1,afk11/secp256k1,libbitcoin/secp256k1,afk11/secp256k1,libbitcoin/secp256k1,bitcoin-core/secp256k1,bitcoin/secp256k1,pmienk/secp256k1,bitcoin/secp256k1,pmienk/secp256k1,bitcoin-core/secp256k1,bitcoin/secp256k1,ElementsProject/secp256k1-zkp,afk11/secp256k1,evoskuil/secp256k1