Support for paseto v4 (bouncycastle)

[untoldwind]

Addresses #32

There is the still open issue with XChaCha20 that I circumvented by extracting the class from the still open PR in bouncycastle. This should be easily removable once (if?) the PR is ever merged. It should be somewhat save to do this since it only tweaks the parameters for the core algorithm.

It is possible to use libsodium instead, but the required crypto_stream_xchacha20_xor function is not public which would require another rather ugly workaround.