[Experimental!] Add UDP X25519 auth and AES256-GCM AEAD

[utoni]

Note: This is an experimental feature. The PR as you see it here is subject to change at any time. Do not rely on the security of the used cryptographic schemes. Before this will get merged there will be some way of generating new ephemeral keys i.e. by using HKDF (not implemented yet).

It's just an early PR ready to test for #47.