aes: move Block8 to the hazmat module

[newpavlov]

The backends were changed to use a different number of parallel blocks, so the alias is now used only by hazmat functions.

Closes#462

[tarcieri]

I guess I'm confused why the non-hazmat APIs use 9 blocks instead of 8, but this otherwise seems fine

[newpavlov]

See UPD to this comment. It has shown a better performance on several (AMD) CPUs which I've tested the code. There may be issues with my microbenchmarks, but we can easily change the numbers in future based on new data.

I guess a power-of-two parallel block size may be better for chunking, so we need to test this.

[tarcieri]

It would probably be good to test the impact on full AEADs. Seems like having an odd number of blocks is going to make optimizing parallel pipelining (which we don't do, because it makes things slower already) that much harder: RustCrypto/AEADs#74