aes: expose round function

[tarcieri]

Exposing the AES round function is of course another level of cryptographic kryptonite beyond the raw block function, but we do have a valid use case.

Deoxys would benefit: RustCrypto/AEADs#311

I think it's possible to build an abstraction for it over the various backends we have.

[zer0x64]

A quick note here that it would also be pretty useful if we want to implement AEGIS one day