ChaCha20 implementation in ELisp.

Install it from Melpa or clone and install manually, then:

1. (require 'elchacha)
2. (elchacha-encrypt-decrypt key nonce data)

Note that the implementation is obviously slower than with compiled languages and, for example, running an elisp-manual-21-2.8.tar.gz (2455995 bytes) through elchacha-encrypt-decrypt took 30s while with OpenSSL or PyCryptodome it took under one second.

While there might be performance bottlenecks in the current implementation, if you are looking for speed, there are better and safer implementations.