Migrate to stabilized x86/x86_64 SIMD

[sunchao]

Currently the SIMD feature requires the x86intrin crate and a compile time flag RUSTFLAGS="-C target-feature=+sse4.2". Since x86/x86_64 SIMD is already stabilized, we should no longer need this crate dependency, and more importantly, enable feature check at run time via the command: is_x86_feature_detected.

[sadikovi]

That is great! I am curious if there is a nightly release with this PR already. Will update my Rust version!