This implements a fix that closes #14 (removing the unconditional enabling of avx512 features)

It should be noted during my cursory audit, I found this crate appears to hard baseline on x86-64-v2 (with sse4.1 being silently asserted), and there might also be hard baselines on neon+aes with aarch64

This violates typical expectations (that code will compile against target baselines and only enable features that have a positive is_X_feature_detected call) so either compile errors should be inserted if not compiling against those baselines (ala simd_json) or new polyfills/checks should be inserted for software implementation