feat: add AES hardware acceleration for ARMv8

[Blake-Haydon]

This PR adds AES hardware acceleration ARMv8. This is sometime called neon and has a significant speedup on ARM based machines.

Experimental results for my ARM based machine (DPF of size $N=1,000,000$):

• Software AES: 9.23 seconds
• Hardware AES: 0.49 seconds

Roughly a ~18x speedup with no performance degradation on non-ARM machines.

[myl7]

The PR adds a RUSTFLAGS to improve the out-of-the-box performance, which should be enabled by the aes crate.

But I doubt whether there is a high requirement to embed the flag in this library?
RUSTFLAGS is passed to dependencies so users can alternatively set the flags in their own project (in their .cargo/config.toml, or even setting the env when compiling), which provides more flexibility.
Moreover, RUSTFLAGS like CFLAGS, and unlike features, is harder to be composed. With the flag always on, users who do not want the ARMv8 part can not easily exclude it like --no-default-features.
Finally, the ARMv8 part requires Rust 1.61+, which can be a problem for some old distros, e.g., Debian Bullseye (which is at 1.48).

All in all, I think it is better to require users to set the flag when using the library as a dependency.
I can add some description in the readme to help it.

[Blake-Haydon]

Very true, adding a comment in the docs is a better approach. I thought that it was worth bringing it up as it is a significant performance improvement for ARM based users.