Make AVX512IFMA opt-in backend

[pinkforest]

Closes #694 #635 #693

Provides cfg(curve25519_dalek_backend = "unstable_avx512")

Errors

• If nightly is not in use -> Must use nightly in order to use it (similar to compiler main when attempting to use unstables)
• If not x86_64 simd capable target -> As with simd error out similarly

Note: I avoided whole lot of any(curve25519_dalek_backend = "simd", curve25519_dalek_backend = "unstable_avx512") gating because compilre allows multi-value per cfg-key where we emit "simd" in addition to unstable_avx512 when potential use validated giving AVX2 fallback through simd.

If it's desirable to instead have explicit gating then let me know - but it pollutes a lot of gating.

[rozbb]

Thank you! Left some small nits here and there

[rozbb]

Ok this seems good to me, thoughts @tarcieri ?

[rozbb]

Thanks again!

[nazar-pc]

Can this be reverted considering AVX512 was stabilized in Rust and AVX10/256 is no longer a thing. The impact of this change is very substantial on modern AVX512-capable CPUs.

[tarcieri]

@nazar-pc a simple revert won't help, since that just goes back to nightly autodetection, which is undesirable.

The code instead needs to actually be updated to support stable Rust, once 1.89 is released, I believe.

[nazar-pc]

Since unstable_avx512 wasn't included in any of the published releases yet, reverting it seems like the first step towards stable support anyway since it would only cause more churn. Not sure how much of the original justification is still relevant with AVX512 being already stable in the nightly channel.

Requiring unstable_avx512 configuration option is a lot less friendly, so I'm worried about the next release dropping performance on modern CPUs to almost half of what it was previously (for those who use nightly of course).

Maybe extending feature check to nightly or Rust 1.89.0+ instead could already provide a future-compatible solution, I don't think waiting for stable to be out is a blocker for this.

[tarcieri]

Autodetecting nightly is antipattern that has caused massive ecosystem breakages (see e.g. ahash). We had several people from rust-core lecture us that we should not be doing it