Rust 1.27 stablized a variety of cpu intrinsics, including SIMD on x86/64 platforms. This commit moves to using those intrinsics for the optimized Float4 implementation. This means Psychopath now compiles on stable Rust with all optimizations. Yay!
We take a small performance hit for this, but given that it's making things meaningfully more correct I feel like it's more than worth it.
