Simplifying the Async Crypto

• Simplifying the Async Encryption & Decryption with Class methods on their respective objects..
– After using it for awhile, it felt to me that I can actually go ahead and have the steps to carry out the encryption/decryption for larger files async. inside `RNEncryptor` & `RNDecryptor`..

Overall, I think its just a basic solution to it but we can enhance it based on your feedback! :)