This is the same thing we do in encrypt

This saves around 6% on my machine.

Here, this gives around 2% in speedup in the 1024 byte case.

This saves the integer conversion. Here, for the 1024 byte benchmark, I
get an improvement of ~4% in runtime

I just want to ensure that we get the same encrypted values as the
reference (which seems fine), but for some reason, I get a lot of
crashes in aez:

AddressSanitizer:DEADLYSIGNAL
=================================================================
==15467==ERROR: AddressSanitizer: SEGV on unknown address 0x7b34b0420000 (pc 0x6371fcd8f682 bp 0x7ffceb91abf0 sp 0x7ffceb91a950 T0)
==15467==The signal is caused by a READ memory access.
    #0 0x6371fcd8f682 in _mm_loadu_si128 /usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/include/emmintrin.h:706:10
    #1 0x6371fcd8f682 in loadu /home/daniel/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/aez-0.0.7/aez5-impls/aesni/encrypt.c:107:46
    #2 0x6371fcd8f682 in cipher_aez_core /home/daniel/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/aez-0.0.7/aez5-impls/aesni/encrypt.c:572:32
    #3 0x6371fcd8d581 in aez::Aez::encrypt::h56048920113a17d9 /home/daniel/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/aez-0.0.7/src/lib.rs:118:13

The crash

It doesn't matter much because we barely expect tau > 16, but if
somebody decides to use aez as a way to generate a lot of pseudorandom
bytes, then oh well.

With this change, we make better use of SIMD block xor'ing if available.