Add ellswift usage example

This should hopefully be useful as orientation for users implementing
the key exchange 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).

README.md

@@ -114,6 +114,7 @@ Usage examples can be found in the [examples](examples) directory. To compile th
   * [ECDSA example](examples/ecdsa.c)
   * [Schnorr signatures example](examples/schnorr.c)
   * [Deriving a shared secret (ECDH) example](examples/ecdh.c)
+  * [ElligatorSwift key exchange example](examples/ellswift.c)
 
 To compile the Schnorr signature and ECDH examples, you also need to configure with `--enable-module-schnorrsig` and `--enable-module-ecdh`.