Implement 32- and 64-byte block

[qnfm]

16-byte width is common as is aes block size, but the newer chacha20's state size as well as the sha3-512 output length is 64 byte. So, is there any plan for 32-/64-byte version block() ?

[lukechampine]

I recommend using fastxor.Bytes for that. The only reason I added fastxor.Block is that, in my benchmarks, 16 bytes was right on the threshold where the overhead of calling into SIMD asm outweighed the benefits. (Granted, that may not be true on your machine -- if you want to be sure, you'll have to benchmark it yourself.)