aead: add FromBlockCipher trait?

[newpavlov]

Unfortunately we can't rely on From<BlockCipher>, see #217.

[tarcieri]

I'm not sure this trait is necessary/helpful as it seems From is sufficient to solve the problem of converting a cipher into an AEAD?

Here's an example from the aes-gcm crate:

https://github.com/RustCrypto/AEADs/blob/4fe07ab/aes-gcm/src/lib.rs#L203-L224

[newpavlov]

It works only because the implementation is not generic over block cipher. If I remember correctly the MAC issue, your code will not work for a generic GCM implementation.