switch from cbc to gcm

I also changed the way nonces and padding works. Padding is taken care of automatically by the AEAD interface from what I can tell. The nonce now gets prepended to the ciphertext. To create type safety, I made a new type 'Ciphertext'. (Since a ciphertext is not just a byte slice, it's a byte slice with a specific format, IE a nonce prepended at the beginning).

I was going to include functions for readers and writers but I actually wanted to work more on the renter first before doing that. One concern is that if we're encrypting a 1TB file, we have to find someway to encrypt it and then upload it. I'm not sure what the best design there is but I want to make sure we're not using the disk too much.