Pure Elixir RC4 stream cipher.

Output matches :crypto.crypto_one_time(:rc4, key, data, _encrypt?) on OTP builds whose OpenSSL still exposes RC4. RC4 is symmetric: encrypt and decrypt use the same XOR keystream, and the encrypt? flag does not change the result.

Intended as a fallback when OpenSSL 3 omits RC4 from the default provider.