Add ellswift usage example

[theStack]

This should hopefully be useful as orientation for users implementing the shared secret derivation part of BIP324. Conceptually the example is not very different to the ECDH one, so a lot of code/comments are just copied (e.g. context creation, secret key generation, shared secret comparison, console output, cleanup with secret key clearing).

[theStack]

@real-or-random: Thanks for the quick review, force-pushed with all the suggestions taken.