Enable SIMD by default

[coder543]

Now that 1.27 has landed, it would be awesome to see SIMD enabled by default. Currently, this section exists in the README:

Usage: enable SIMD optimizations

This crate provides an unstable feature that can only be enabled on nightly Rust. When this feature is enabled, the regex crate will use SIMD optimizations if your CPU supports them. No additional compile time flags are required; the regex crate will detect your CPU support at runtime.

When std::arch becomes stable, then these optimizations will be enabled automatically.