Handles variable-length nonce

[masihyeganeh]

I'm trying to fix #65, but I'm not sure I'm on the right track.
This PR changes nonce type to &[u8] instead of fixed-sized GenericArray.
The main code to handle variable-length nonces will be in RustCrypto/AEADs.

[tarcieri]

I like the idea of it being possible to use a slice for a nonce (this is a blocker AES-SIV too), but I don't like it being unilateral, because it introduces potential errors when using AEADs with a fixed-size nonce (which is the case 99.9% of the time).

I think it might make sense to make the nonce an associated type. See the sketch of that idea I suggested on the original Aead trait PR:

#40 (comment)

[masihyeganeh]

I like your solution better. I just not exactly sure how to do it.
This variable-length nonce feature is holding my project back. I am willing to help implementing it.
Just let me know if I can help you with this.

[tarcieri]

It seems RustCrypto/AEADs#126 takes care of supporting any-sized GCM none without changing the trait.

This also seems like a general solution to the problem (i.e. making Aead::NonceSize generic) so I think that's actually more desirable than trying to support slices as nonces.